Not looking for a solution as I'm sure that would involve AT&T. Just referencing here in case anyone else comes across. Thanks to @leejor for pointing out voicemail in this post https://www.3cx.com/community/threa...n-external-3rd-party-declines-the-call.54413/ On at least US AT&T cell phones that do NOT have voicemail set up, which is where a call would normally be sent when you decline a cell call: Call cell from desk phone,decline call on the cell, desk phone gets at&t message “wireless customer… is not available”. But if I don’t hang up desk phone after hearing this message, the cell rings again with incoming call. Cycle repeats until desk phone hung up. Same behavior with 3CX for Windows in softphone mode. Similar from 3CX for Windows client with makecall to desk phone, only difference is on makecall you never hear the at&t message on the desk phone. Desk phone just sits playing the makecall hold music, meanwhile on the cell side, I decline the call, then 22 seconds later, cell receives call again. Cycle keeps repeating until the desk phone is hung up. If cell has vm set up, then declining call on cell sends to voicemail and that is what you hear on desk phone, which is what you'd expect. No voicemail and cell user declines call, PBX keeps sending call (likely due to AT&T not sending something the PBX needs to terminate the call), meanwhile cell user is repeatedly declining call.