Call sometimes muted in one side for a seconds, sometimes drop

When a extension need to tranfer the call do another extension, one side stay muted for a seconds and ok.
Other times one side stay muted for a seconds and drop the call.

What can be happening? Thank you

Please at least tell us what channel technology you are using. You will also probably need to provide verbose level 5 logging and the channel technology specific debug logs at at least that level.