Calls that ring to the coverage key should continue to ring on the covered phone as well. Those type of calls should not forward to voice mail unless the covered phone is in a forwarded state,( could be preset) or the co line is preset forwarded. There is something else going on here for sure.

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"Old phone guys never die, they just get locked in some closet with an old phone system and forgotten about"

Retired, taking photographs and hoping to fly one of my many kites.