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Problem with IP phones after ISP failover change

I've ran into a strange problem at one of my clients. They use "shortelesky" cloud IP telephony service and use Cisco 7960 IP phones. The problem is following:

They have 2 ISPs configured on ASA5520

1) MAIN 2)BACKUP

The configuration is done through IP SLA tracking. The phones do work when the connection gets swapped from MAIN to BACKUP and i see routes 0.0.0.0 are pointed to IP of BACKUP ISP IP. So i would get:

C BACKUP ISP IP 255.255.255.248 is directly connected, BACKIP_ISP

C MAIN ISP IP 255.255.255.248 is directly connected, MAIN_ISP

and this part when MAIN is up:

S* 0.0.0.0 0.0.0.0 [1/0] via MAIN ISP IP, MAIN_ISP

and when BACKUP gets switched i see route is being updated:

S* 0.0.0.0 0.0.0.0 [1/0] via BACKUP ISP IP, BACKUP_ISP

Once the interfaces get switched back to MAIN everything does work the route S* gets populated with MAIN ISP IP in the 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 to have "C BACKUP ISP IP 255.255.255.248 is directly connected, BACKIP_ISP" removed and then added. This seems to work, we also need to reinitialize the phones, meaning since they are PoE we need to unplug/plug patch cable.

Please if anyone knows what can I do about this or what direction to look into, I would really appreciate. The main issue here that on the network side I have captured the traffic from all conditions and there is no difference whatsoever when phones work and when they do not.

Thank you.

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