Keeping phone number form BT to Plusnet

A family member currently has BT Infinity and phone calls with BT. They are now considering a move to FTTC and phone with Plusnet but don't want to lose their phone number.

Plusnet says

Whenever possible we'll try and keep your current phone number. However, in some cases we can't transfer numbers.

Could anyone give any more information on this and if they place an order online will their phone number move automatically?

Thanks.

My telephone number moved without any issues about 10 months ago. Moving from BT phone + Infinity to Plusnet phone + 40/20.

I think complications are most likely if LLU is involved, which makes BT/Plusnet changes (either way) reasonably safe.
However, no promises or expert knowledge, just on that went OK.

If it is a BT number and has always been a BT number, it will be fine. If the number has been ported to BT from somewhere else, there may be an issue.

Staff member here.

Typically we'll contact you if there's a problem retaining the number rather than just switching it to something else. Though I don't think I have ever seen an issue with a BT - Plusnet migration in my time provisioning.

Matty