Welcome to the community!
If you wish to keep the existing phone number from old ISP, we recommend to keep the service activate until the porting progress is done. Once the existing phone number is ported you may contact your ISP to cancel the service.
But if you do not want to keep the existing phone number, TPG will provide you a new home phone number for the service, then you may proceed the cancellation of the service.
Let me know should you require further assistance.
Do I need to cancel my old ISP before installing my TPG modem? What is the best method for this, please?