Cisco 8851 registration issues

I am working with Asterisk 1.8.32 and have some cisco 8851 phones and 2 PBXs. I have configured the phones to connect to both PBXs by adding 2 entries to my callManagerGroup. When I first started everything I verified they successfully registered with my first PBX and were able to make calls. Next I wanted to verify they would register with the second PBX so I shutdown my first PBX and started asterisk on the second PBX. The phones don’t seem to register.

After much testing and debugging I noticed that if I reset one of the phones (Phone A) while PBX2 is running asterisk the Phone A registers successfully while Phone B still doesn’t register. If I shutdown PBX2 and start PBX1 back up Phone B registers successfully with it while Phone A won’t register until I reset it.

Through a wireshark capture I was able to see a Sip registration event sent every 2 minutes (which is what my timerKeepAliveExpires is set to) where it would have an entry under the contact that said expires: 0. Through research I have found where a Sip Register command with a parameter of expires set to 0 is used to unregister.

Why is the Phone constantly resending unregister commands to the second PBX if the first PBX it was connected to goes away? Is there any suggestions on how to resolve this?