You have two options when it comes to wireless phone numbers.
1. You have the option to port/transfer you existing phone number from your current carrier.
This is only possible if the phone number is still active and the account has been in good standing. When signing up with a new service plan, most carriers will ask you if you have a phone number you want to keep. In the event you do wish to keep you phone number, simply select the option to do so.
You will typically need to provide account information from your previous carrier such as your pin number, social security number, and the phone number you intend to keep. Once this information has been entered, the number porting process will begin.
At that point, your new service provider will notify your previous carrier that the phone number has been switched. Your previous carrier would then release the phone number to your new carrier and is ready for use.
It may sound complicated but its a really simple process - most carriers will offer this number porting as part of the online activation process when you have received your new sim-card - some will also manage the process over the phone via their Customer Services team
2. The second option for wireless phone number is to receive a new phone number.
During the sign up process, you new provider will typically give you the option to keep you old number or receive a new phone number. In the event you are in need of a new phone number, simply select the option to receive one. Once you've completed all the requested information, you'll be issued a new phone number. Your new number will be associated with you new SIM card.
Here's a link to our Switching and BYO Phone to a new carrier guide for more information