Once a flow starts using an uplink the MX keeps it on that uplink unless it fails. So when you fail over to your secondary link the flow gets moved, but when you failback it will keep using that secndary link.
For phones that mostly use UDP this creates issues, as their is both SIP and RTP flows - which are seperate.
Personally, we use SIP/TLS for all our phones - and this works flawlessly for us (different brand of phone system though). This uses TCP, and this has a clear start and end of flow, and stops one way audio call issues.
If you can't use SIP/TLS then try using SIP/TCP.