Plug the cord from the offending line into a different jack on the back of the phone (use the jack for Line 4, for instance)

This will let you learn if the trouble is in the multi-line phone, or in the service coming in.

Testing at a demarc is not as simple as the telco would have us believe. A telephone company dial tone restoration after a trouble is cleared can be delayed by certain maintenance protocols. You often need to wait several minutes, or ring into the line, to reset the circuit in some of the older electronic switching systems.

Arthur P. Bloom
"30 years of faithful service...15 years on hold"