3CX 12, Cisco SPA504 handsets. They complain that the voicemail light is on.. when it should not be. More so, we set the extensions to deliver to email and delete from the mbox.. so that indicator should be about never. When they check for voicemail, naturally they have none in the system. After vacation and I was back last week.. my own SPA has it lit on our own PBX.. no vm is there. So.. I fired up WireShark to see what's going on. First.. I set the test extension (307) voicemail to just drop onto the PBX.. no email. Used a softphone to dial *4+307.. left a voicemail. Light comes on. From the same softphone.. dialed voicemail, backed out and checked voicemail for 307.. light turns off. Indeed.. there is a NOTIFY message via SIP from the PBX to the handset per event: Left a voicemail: Messages-waiting: yes voice-message: 1/0 Heard the voicemail: Messages-waiting: no voice-message: 0/1 Deleted the voicemail: Messages-waiting: no voice-message: 0/0 Okay.. fine. That's "how" the indicator works.. but how about when an extension is using the option to deliver voicemail via email and delete from the mbox? I expect there are no notifies sent to an endpoint.. apparently that is not the case. Left a voicemail: Messages-waiting: yes voice-message: 1/0 And.. immediately after that notify (.000132 seconds on the trace..): Messages-waiting: no voice-message: 0/0 On that test.. I did see the light "blip" on. Seems to be a bug? I have multiple clients on various revisions of 3CX.. but plenty on Cisco. Other than "mine" and all of theirs.. haven't heard too much otherwise. My guess thus far is that the Cisco may not be getting the second NOTIFY, cannot process it fast enough from the last, or sees an anomaly and drops. Whatever the case.. the result is the indicator stays on.. the only recourse is to reboot the phone. Changing the MWI setting to No will trigger the reboot anyway.. Ideas? I'll be rigging more wireshark when I can.