There’s an easy way to get zero roaming fees, inexpensive internet service and cheap phone calls when you travel. The trick is the SIM card, a tiny, removable chip, inside your phone.

Activation fee: $15 plus $4.99 SIM card charge iPhone 7 (32GB): $398.99 on a two-year plan (on the Platinum Plan) Samsung Galaxy S7 (32GB): $329.99 on a two-year plan (on the Platinum Plan) ...

If you've already got the phone you want, SIM-only deals tempt you with cheap rates for just the SIM card -- but your phone will have to be either unlocked or locked to O2 in order to use this option.