All three major carriers have some form of Port Protection (AT&T), Number Lock (Verizon), Takeover Protection (T-Mobile). Make sure that you unlock your number with your existing carrier before submitting the number transfer request. See below for additional guidance by each carrier.
Note: Many times port protection is only removed for a limited amount of time and/or the port out PIN may change to allow the bypass of port protection.
Please confirm with your current carrier:
How long the port protection is removed for?
Has the port out PIN changed?
AT&T:
If you have Port protection on this line, you'll need to contact AT&T Porting Department at 888-898-7685 to remove it before you can transfer the phone number.
Verizon:
The number transfer PIN must be used within seven days. You must request a new PIN if it has expired. If you have Number Lock on this line, you'll need to remove it before you can transfer the phone number. On the myVerizon website, go to the Number Lock page, select Off for the number you want to port, respond to the security authorization and finally tap Save Changes.
T-Mobile:
If you have Takeover Protection on this line, you'll need to contact the T-Mobile Porting Department at 1-877-789-3106 to remove it before you can transfer the phone number.