VoIP problem with stack SIP with the MX WAN connectivity failover

Dario_DG_ITA
Here to help

VoIP problem with stack SIP with the MX WAN connectivity failover

Hello All.
I've ran into a strange problem at one of my clients.
They use cloud PBX IP telephony service in internet and use  SIP IP phones in customer site.
The problem is following:
They have 2 WAN link configured on Meraki MX FW 
WAN1:MAIN - WAN2:BACKUP

The phones do work when the connection gets swapped from MAIN to BACKUP.

Once the interfaces get switched back to MAIN everything does work, routing table, WAN for workstations and phones is reachable, but:
When i dial after a swap from FAILOVER to MAIN to any phones, the phone does dial out but we hear no voice, I do get the call to my cell phone, but i hear no voice neither on the IP phone or on receiving cell phone.
The only way to resolve this problem as of right now which is very annoying is to disable/enable BACKUP interface. 

Please if anyone knows what can I do about this or what direction to look into, I would really appreciate.
I think that the problem is the stack sip ...

Thank you.

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