Telephone line identifier

What is the unique identifier for a telephone line ?

I appreciate that we usually identify a line by referring to the telephone number attached to it, but telephone numbers "move", so there must be a unique identifier for each physical line.

Comments...

Do you mean the lines ‘routing’ ?

From exchange bar pair to cabinet number, E and D side pairs. FTTC ties if routed, DP ID, and pair ?

The above is reasonably flexible too

I can't speak for full MPF, but at least with Openreach it's the access line ID. This is a number I've been able to put into the BT Wholesale broadband checker regardless of ISP and get the details on things like highest sync rate.