S50 - Poor performance when handling multiple calls

Hi, we have a S50 which we have used without issue for a couple of years. Recently we are experiencing issues when the system is handling more than 2 calls, (poor audio quality and dropped calls) in terms of networking no changes have been made prior to the issues starting and there's more than enough bandwidth available. (full fibre 500/70mbps) 

Issue possibly started when the S50 updated to the latest firmware but not 100% sure. 

Does anyone have any suggestions on what to try?

Thank you