There isn't a number to call, if you need assistance you need to contact customer service via the available channels, look in 'Get Help'.
But for porting the process is generally to go to the new provider and start the process there, signup, request a port. Keep your account at Public active, keep your Public SIM active in your phone until you respond to the confirmation text message for the port. Note that after porting your account will be closed immediately, service will stop, rewards will be lost. So download any logs or receipts you need.
