We have a UC520 with 8 ports of analog lines - all ring to a 7961 reception desk that does not experience the problem. When the receptionist is away calls are answered via GroupPickup by 7941 phones. every 3rd or 4th call answered this way experiences one way talk path. The internal users cannot hear the outside caller, but the caller can hear the internal user.
Telco reports that the lines are fine.