Intermittenantly, callers on the MTE experience a brief 2 to 4 seconds when they first pick up the phone where the caller can’t hear them, then it clears up. Happens with almost all tenants, if not all.
Any ideas on where to look for the root cause?
Yes
For us it was linked to the workload of the server. We had 1200 phones registering every 60 seconds, on top of call traffic. We resolved this by moving to a different router type (DD-WRT on an Asus 520-WL) which allowed us to define the UDP timeout and thus set the re-registration to something higher than 60 seconds (which our first router choice would destroy 'unused' UDP sessions after about 60 seconds). By up'ing the registration period we reduced the workload for the server and these call delays disappeared.
This is a very unusual symptom and could be stemming from many different root causes so don't take this as an absolute answer, just something to take a look at.
I am experiencing the same thing here.