Bringing phone number to plusnet

[#1]

I'm currently with talktalk and I'd like to transfer to plusnet. However, when I start the order process on the plusnet website I am told that my existing landline phone number can not be retained. My understanding is that porting numbers between providers should always be possible. Is there anything I can do to explore further what is the problem, or do I just give up and stay with talktalk? (Changing my number isn't an option.)

[#2]

I've had the same landline number from the GPO, to BT,02, Sky to Plusnet.

Plusenet told me I couldn't  keep it too, but that was an error, so go back and get some confirmation.