My cellphone provider just billed me $600 more than expected. Can they do that?

Maybe — but not without limits. Your provider can bill you for roaming you actually used, but national rules cap certain charges and require your provider to warn you before costs pile up. If those rules weren't followed, you have a strong basis to ask for the bill to be reduced.

It helps to know what kind of charges add up when you travel. Using your phone outside your provider's network area is called roaming. You can be charged for the calls, texts, picture messages, and data you send or receive while roaming.

The Wireless Code — the national rules every carrier in Canada must follow — gives you one key protection here. Your provider must stop charging you for data roaming once those charges reach $100 in a single monthly billing period, unless you agree to pay more. This $100 cap applies to your whole account, no matter how many devices are on it.

Two things to keep in mind:

• The cap covers data roaming only. Charges for voice calls and texts while roaming are not capped, so those can keep adding up.

• If you keep using data after your provider tells you you've hit the cap, and you agree to pay more, the charges continue.

(A separate $50 cap applies to data overage — going over the data limit on your regular plan at home. That's a different situation from roaming.)

Your provider also has to warn you, at no charge, when your device starts roaming in another country. The warning must tell you the rates for voice, text, and data. It must go to both the account holder and the person using the device.

If you think your bill is wrong, here's how to dispute it:

• Ask for an itemized bill, then contact your provider. An itemized bill shows what each charge is for. Calmly explain the problem and ask them to lower the bill. Try to reach someone with authority, like a manager. Keep a record of who you speak with and when.

• Send a complaint letter. Explain why you dispute the bill and which charges are wrong. Include a copy of the bill, your contact information, and your account number.

• File a complaint with the CCTS. The Commission for Complaints for Telecom-television Services administers the Wireless Code and handles consumer complaints about phone service. You'll need to try resolving the issue with your provider first. Then you can submit your complaint to the CCTS.

Our page on disputing a phone bill includes a template letter you can adapt to your situation and send to your provider.