As I said before... the guy on the phone was bs'ing you. Mine went like this:
NUMBER1= SIM Only 1 month simplicity for smartphones, number to keep.
NUMBER2= SIM Only deal from this thread with quidco cashback, number to lose.
Rang up. Guy puts in request to different department to port my number.
Ported over NUMBER1 onto NUMBER2 SIM. Took a few hours with the turning on and off thing as you said.
Done.
You just got a complete jerk on the phone who didn't know what he was talking about.
Yeah I knew they could do it tbh. He so insisted he was right and accused me of trying to beat the system. I should have got his name.