English to German Tech/Engineering

Context: telecommunications. An underlying carrier is a telecom company (often a large, established one such as, e.g., AT&T or Sprint) that leases transmission line capacity (copper or fiber) to other carriers who then, in turn, make this leased capacity available to their own customers.

What I’m looking for is the German terminus technicus for “underlying carrier” - if there is one.

The specific context is as follows: “[The company] will obtain dark-fiber capacity from an underlying carrier so that the transmission line itself does not fall into the area of responsibility of [the company]. [The company] will cooperate with underlying carrier to ensure that the transmission lines and the termination points at the end users are sufficiently protected by this carrier.”