I am with Sipgate with whom I am very satisfied. I am about to port my landline number to them as my contract is up. Sipgate say that it is no problem to port my number again to anather provider. Question is "is this only to another VOIP provider or could it also be to a landline (e.g. BT)?
you may find it hard to do as analogue lines are been ceased by 2025, BT will stop selling analogue lines in 2020
at some point in the near future you will be able to get a broadband only connection fibre of copper.
once this is available you will see providers moving people to VOIP on a Broadband only connection.