Overseas charges: If overseas use is not in your phone plan, you'll pay extra to make calls, send texts and use data. These charges can be much higher than you expect. Make sure you understand the cost and any limits on use.
Charges for going over data, minutes and texts: When you go over your monthly limit of texts, calls and data, you are often charged at much higher rates than you're used to. Pay attention to how much you are using and what the rates are. If it happens regularly, upgrading to a bigger plan might be cheaper than paying excess fees each month.
Expired discounts: When you sign up for a new plan, there can be joining deals with discounts, free add ons and other perks, eg 50% off, bonus data, free subscriptions to streaming services. Check when these discounts finish to avoid getting nasty surprises on your bill.
Charges not covered on your plan: Check your terms and conditions to see what is covered, eg voicemail calls.