You can choose to keep your existing phone number or get a new number from TPG. Preliminary checks will be done to see if porting is available to you. Installation may take 10-20 working days. PM me your contact number so we can assist you further.
Are you looking at NBN or ADSL2+ with TPG? There are slightly different installation timeframes. You can check what is available for you by entering your address at https://www.tpg.com.au/adsl.
It's best to cancel your iiNet service only after your new TPG service is installed, especially if you will be porting or transferring your home phone number from iiNet to TPG. TPG will be unable to port your home phone number from iiNet to TPG if it is disconnected. Cancelling iiNet after the TPG service is installed will also ensure you don't have any gap of days with no internet service