Several users in the same Call Manager group reported that they receive a fast busy tone when they go off hook. They reported that the issue went away after a few minutes. Where should the voice…

A SIP gateway issue would affect calls routing through that specific gateway, not all phones in a Call Manager group at the off-hook stage before any call is placed.

When all users in a Call Manager group experience the same symptom simultaneously, the common element is the CUCM subscriber to which those phones are registered. A subscriber that is overloaded, restarting, or temporarily unavailable will cause registered phones to receive a fast busy tone, and the issue will self-resolve once the subscriber recovers or phones re-register to a backup node.

A failed network cable would affect only the individual device connected to that cable, not an entire group of users.

A single IP phone model issue would not simultaneously affect multiple users across a group; this would only be relevant if one user reported the problem.