Preservation Mode on IP Phone – What are causes for it?

[Bill]

I know the simple answer is when the phone loses communication with Call Manager.
But it seems there other conditions where this can happen.

I have a condition where every time I make a call to a particular SIP provider, the phone goes into Preservation Mode.
I do have issues with calls connecting to this SIP provider, which I believe are firewall related.

But why would that make the phone go into Preservation Mode?
It happens every time, all the while CUCM can ping the phone.

I don’t believe this is an issue with communications between the phone and CUCM.
Phone and CUCM are on the same subnet – no firewalls between them.
I’ve tried cranking up the rtcp inactive timers just to see if it would make a difference – it did not.

This is CUCM Version 12.5.1.18100-14
The phone is an 8845.

Thanks

[Ibrahim AlQumairi]

Please check the routing path between the IP phone that you made the call from and the voice gateway router that is connected to the ISP's SIP trunk. The actual path for the call is from the IP phone to the voice gateway, so please try to identify what is blocking the media traffic (RTP) in between. I would also like to caution you that a successful ping does not necessarily mean that the media traffic is flowing without any obstructions.

[Bill]

Ibrahim
Thanks for the reply.
I am sure nothing is blocking the path from the phone to the local gateway (my gateway).
I am also pretty sure the RTP media path is not working to the remote SIP gateway (the service providers).
I know I have a firewall issue to the provider. 
Would this cause the phone to go into Preservation Mode because of no RTP Packets ? 

[Michalis Papasavva]

Hi Bill,

Did you find a solution for the issue?