What I meant by seamless was avoiding a time when I had no service at all. As far as I can remember I get a PAC number from the old supplier whilst still with that supplier. Then I join another supplier and give them the PAC number to retain my old telephone number. The new supplier sends me the new SIM - as soon as I put the sim in the phone and activate it the old supplier cancels the contract. Is that how it goes?
The number port can take up to 24 hours to fully complete. There may be time during the transfer, where the number may appear to have fully transferred but you may find that can't either receive or make calls/texts or use your data. This a part of the porting process and not a fault.
Thanks so much, Enlii - very clear and helpful. When I take out the contract with the new supplier do I start paying for it then and there, whilst I'm still paying for the old supplier, or is it on the transfer date that payment to the old ceases and the new begins?