Keep You number
If you've decided to switch carriers, and want to keep your phone number, it's important that you do not cancel your existing service before opening your account with your new carrier. To ensure that you are able to maintain your number, inform your new carrier when you start the service that you want to port your number over from your current provider.

Your new carrier will then do the legwork for you by informing your old carrier that you're making the switch. The transfer itself will happen automatically, though it may take anywhere from a couple of hours to a couple of days. To cover your bases, you may have to carry around both your old smartphone (the one locked to your original carrier) and your new one 'til the switch is complete. When your old one stops working and your new one starts ringing, you'll know the process is complete.

The Cost of Switching
If you want to keep your number when you switch to a new carrier, you may have to pay for the privilege—then again, you may not.

According to the FCC, carriers are allowed to charge a fee to customers at the time their number is ported. However, there are no rules preventing a new carrier from paying an old carrier's porting costs for the benefit of the new customer.

Most mobile operators will cover the cost of the switch as a means of enticing new customers. When you switch, be sure to ask your new carrier whether it has a policy of paying or reimbursing such charges.