I would suggest to enable to following settings:
Check the service port running for your SIP phone if it's not running using default service port.
Enable QOS ---> Application for the SIP services - Set priority to high
Configure SIP service for WAN fail-over instead of Load-balancing:
Outbound policy for SIP Phone/Service
P/S: Make sure new created SIP outbound policy is put on top of all the existing outbound policy.
Please monitor the SIP service quality after added the above settings