Intermittent no audio on handset but speakerphone still works

[ZPM]

Having a strange issue that seems to bounce around the office, users report no sound on handset when receiving a call BUT if they switch to speakerphone it works. The only fix is restart the phone. Seems to happen 1 or 2 times a day but is intermittent. I have tried rolling back firmware and using the latest and even replaced the phones with brand new ones. Wondering if this could be a networking issue just weird that it works on speakerphone. Any tips for troubleshooting?

Phones: Yealink T48s (Total 14 phones)
ISP: Spectrum cable / in bridge mode (SIP ALG disabled)
Router/Firewall: Sonicwall
Switch: HP PoE switch

FusionPBX: 4.4.10
Switch: 1.8.7
OS: Debian 9
Virtual machine 2 cores 4 GB memory

 

[JamesBorne]

switch to speakerphone it works

I would be quick in saying speakerphone doesn't do any SIP action - just switches the sound destination.
Perhaps there is something forcing the call to "headphone" mode where no sound comes through the receiver?

 

[ZPM]

So for testing this also affects the headset port as well, phone will answer on speaker phone no problem, if we restart the phone it works normal but seems to be an issue that bounces around the office eventually returning to the same phone.

I also upgraded to master but same issue.

 

[ZPM]

I just do not understand this issue it is like off hook is dead, if I have them remove the handset and hit the off-hook button nothing happens on the display from hitting it. The handsets are connected to the correct port.

 

[ZPM]

Could it be the phone not getting sufficient power? They are using a PoE switch, maybe I try adapters for testing.

 

[JamesBorne]

I've seen phones powering down and screen artifacts when there is too much draw over PoE. Might be switch as you mentioned.