Audio issue when server got disconnected on the network

Hi Guys,

Anyone who can shed a light on our issue with our SIP phones/FreePBX?
Currently, we do have Cisco SPA 303(3 lines)+ others. We have just noticed that each time our server got disconnected from the network and went back, our SIP phones seems to have experienced an audio issue (either a one way or no audio at all). we have to reboot our SIP phones multiple times until audio works back.

Please help. Any reply is much appreciated.