When you transfer your number to your plusnet, do you keep the 30 day minutes/data/SMS and credit on your account? Just bought a sim, so now waiting for it to arrive. Also, from what I have read, you have to transfer your number when you aren't relying on it working for a couple of days.
Ofcom rules state your number must be ported within 1 working day from when you hand PlusNet your PAC code. However, my experience is now way more than that with no end in sight yet, and there are others on this forum who have similar experiences - so do plan ahead to take into account the "PlusNet effect".
That said, you can keep using your phone though with the number your sim card comes allocated with - and it your credit is unaffected by the number porting so if your happy to use an alternative number for a while, go for it.