Hello, Yes we have run into this same problem with these phones. FYI we have Yealink phones in stock for sale at a very low price. In the past we would roll in good working phones as we have never really tried to diagnose the problem so, I dont have an answer / fix for you other than trying different handsets and or units.

Hopefully one of the guys here on Sundance has a better answer for you! Call or email for quote if you would like, and if we have what you need we can sell to you.

TY Mike Schmidt