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From the end of this month a mobile phone ''cap'' will have to be a cap - in other words, it has to represent the maximum you'll spend each month, not the minimum.

From March next year you won't need a degree in applied mathematics to work out which deal is best for you. Each plan will come with a summary setting out the cost of a two-minute call, a text message and one megabyte of data.

A year from now, if you're with one of the major providers, you'll receive warnings when you hit 50 per cent, 85 per cent and finally 100 per cent of the ''included value'' in your plan. (Smaller providers have until 2014 to do this.) There's a lot to like about the new Telecommunications Consumer Protections Code, which has been rewritten after a deluge of complaints in recent years about shockingly high mobile phone bills.

Natasha Nikolovski.

Hefty sum ... Natasha Nikolovski was hit with a $1600 mobile phone bill after her overseas trip. Photo: Edwina Pickles

''What it means is more transparency for consumers, in advertising, in contracts and in billing,'' says Teresa Corbin, the chief executive of the Australian Communications Consumer Action Network (which, separately, has had its government funding renewed).

''It doesn't close all the loopholes, but it's a significant improvement. They've definitely taken stronger steps than I expected they would.''

New powers for the Australian Communications and Media Authority, making it easier for the regulator to penalise code infringements, are also significant, she says.

That said, there's nothing in the new code that completely rules out getting a killer bill, especially if you have a smartphone with internet access.

SHORTCOMINGS

As it stands, in a year, when you get an alert saying you've used up 100 per cent of your calls, text messages and data for the month, the meter will still keep ticking - at what could be described as penalty rates - unless you stop using your phone or come to an arrangement to move to a higher plan. And under the code, those alerts are allowed to be 48 hours behind - meaning you could chew through two more days of expensive calls, texts and online services before you realise you've exceeded your plan limits.

There will also be services, such as international calls and online access overseas (known as data roaming), that aren't included in your plan to start with. What's more, the advent of 4G/LTE networks has raised fresh concern about bill shock and data overuse for people who have high-end phones such as the recently released iPhone 5 and the Samsung Galaxy S III 4G.

The ACMA says overseas research shows having LTE in a phone can double a consumer's usage of data.

Corbin - who notes that the cheapest iPhone 5 plans include a dangerously low 250 megabytes of data - says she would have liked the new code to replace higher charges for excess data usage with something such as a ''speed throttle'' once people exceed their plan limits. And why should charges for excess usage be higher than standard rates anyway?

ACCAN also lobbied - unsuccessfully - for customers to be allowed to nominate a maximum credit limit on their account each month, just as they would with a credit card.

The Telecommunications Industry Ombudsman also considers data a big worry, saying disputes over internet usage charges tripled in the year to March 31. (More recent statistics won't be available until the annual report is released later this month.)

So what defences should you have in place to ensure you don't get mugged by your phone bill in the year or two before the new rules are fully implemented - and even afterwards?

MONITORING TOOLS

First, choose the right plan. The ACMA puts the cost of consumers choosing the wrong plan at $1.5 billion a year.

Consumers often make the false economy of choosing a cheaper plan, only to rack up high charges when they exceed the limits in the plan.

''If you blow your cap you're going to pay excess charges that are horrendous,'' says the managing director of phone broker Logicall, Michael Starr.

He suggests considering one of the ''really good'' unlimited plans now available (see the table for options). But make sure you know what's included in a plan - the ''unlimited'' may refer just to voice calls, for instance (see ''What to look for in a plan'', left).

Corbin suggests asking friends and family for their experience of network coverage and customer service, because just how good - or bad - those things are won't be apparent until you've already signed up.

Once you're on a plan, use spend-monitoring tools. Jo Ucukalo, chief executive of complaint resolution service Handle My Complaint, says one in three post-paid customers receive unexpectedly high bills but apparently only 10 per cent of us use tools that monitor our phone and internet usage.

Most providers offer monitoring services on your phone or online, although the information may be 24 to 48 hours old. There are also third-party apps such as Onavo and DataMan, though they monitor data only, not voice calls or texts. Some smartphones, such as the Samsung Galaxy S III, have built-in data monitors.

IMPORTANCE OF SETTINGS

Be smart with your settings. The ACMA suggests disabling ''location services'', which allow applications such as Google Maps to gather data to work out where you are.

You can switch email to manual retrieval, rather than having your phone constantly checking it, and give apps ''permission'' to operate only when wi-fi is available.

''Try to outsmart your smartphone,'' Cameron Craig, a director of mobile phone comparison site WhistleOut, says. ''Make sure you're controlling how it gets information.

''Think about what it's trying to do and limit what it tries to do when you're away from wi-fi.''

Wi-fi is your friend and you should use it whenever possible, Craig says.

Have your phone set to connect automatically into your home network, the office network, wi-fi in your favourite cafe or in a local ''wi-fi hot spot''.

Download information such as podcasts and music on to your phone while you have access to wi-fi, rather than use up your plan's data allowance by ''streaming'' them while you're out.

Bill shock is a common experience for people who use their phone while travelling. Voice calls will be at higher rates than at home and you could incur data charges as high as $20 a megabyte for accessing the internet.

TRAPS WHEN TRAVELLING

Turn data roaming off while overseas, says Starr, who has seen roaming bills, admittedly for business customers, as high as $US20,000. ''Avoid roaming at all costs,'' he says. ''Regardless of who you're with, you're going to pay too much.''

Rely on wi-fi for data or ask your provider for an add-on data roaming pack for travellers.

Even voice calls can generate unexpected bills in the hundreds of dollars, so ask your phone provider to ''unlock'' your phone so you can buy another SIM card at your destination and pay local rates. Global SIM cards are another option, perhaps the most visible brand in Australia being TravelSIM.

TravelSIM chief executive Jamien Zimmermann says local SIM cards are a valid choice if you're visiting an English-speaking country with good telecommunications and you're not crossing borders (which would require multiple cards). But one advantage of a card such as the TravelSIM is that you can make arrangements before you leave on your trip (see our case study).

Voice calls are at rates well below what you'll pay on your home plan but Zimmermann acknowledges that getting data costs down has proven more difficult. He notes, however, that a prepaid card such as the TravelSIM does protect you from bill shock because once the credit is gone, the meter stops running. ''It may be that [data use means] your $50 goes quickly, but at least it's $50, not $3000,'' he says.

Finally, assert your rights if you strike a problem.

If you're not happy with the response from the first customer-service person you speak to, ask for the call to be escalated to someone with the power to act, Ucukalo says.

Ask what you can do to lower any excess charges - such as buying a top-up - and don't hesitate to ask for a reversal of the charges if the issue is a one-off problem. They should suspend payment on a bill that's in dispute in any case, she says.

Key points

❏ Choose a plan carefully.

❏ Use spend-monitoring tools.

❏ Be smart with your settings.

❏ Wi-fi is your friend.

❏ Be wary of high charges when travelling overseas.

Out of range

Natasha Nikolovski thought she had "unlocked" her iPhone from her local network so she could insert a local SIM card once she arrived in the US for a six-week holiday.

But after spending half a day trying to buy a SIM card in Las Vegas and eventually paying $US160 ($154) for a prepaid plan, she found her phone wasn't unlocked at all.

Unable to fix the problem, she turned roaming on. "It seemed like the easiest option," Nikolovski says.

But the phone bill after her trip was $1600 - for just voice calls and texts. "Luckily, I was advised to turn data off, so I could only access internet via free wi-fi," she says.

Nikolovski says it turns out the problem was that she used call cards in conjunction with her mobile phone to ring home.

"When you purchase those call cards in the US, you have to dial a number first and follow the prompts. The actual phone calls home - especially to land lines - were cheap, but my roaming got me on [those other calls]."

Her local network knocked a bit off the bill because of the trouble she had unlocking her phone in the first place, but the final sum was still hefty.

For her next trip, she used a TravelSIM card on her now-unlocked phone. "I paid $50 for the card, which came with credit. I also bought $25 in extra credit," she says.

"I'm going on holidays in two weeks and I'll be using it again. I still have most of my credit left over. I'd advise anyone to do this. Avoid roaming at all costs."

Checklist: What to look for before you sign up

There's no such thing as the "best" mobile phone plan because it depends on what you need from your phone.

One user might value having unlimited voice calls, another might favour text messaging, while a third might consider having "free" social networking a must, so access to sites such as Facebook and Twitter doesn't count towards data usage.

If you have a smartphone, you'll want a good amount of data included in your plan so you don't end up paying high fees for excess usage.

If you have family overseas, you might appreciate international calls being included in your plan.

People living outside metropolitan areas might require a provider that uses the Telstra network. Here are some questions you should ask before you sign up, care of ACCAN's guide, Making the Right Call (available at accan.org.au):

❏ How much data and how many texts/local calls are included?

❏ Are international calls included?

❏ How can I monitor my spending?

❏ What's the minimum I'll pay during the term of the contract?

❏ What equipment will be provided and is there any additional charge?

❏ Do you have coverage where I live, work and travel?

❏ What happens if I exceed my download limit?

❏ Will I receive a warning?

❏ How much will I pay to make calls or use the internet when overseas?