FortiFone not registering with FortiVoice 500F over Ipsec Tunnel

[emurray91]

I have two phones that are not registering with Fortivoice that are behind a site-to-site IPSEC VPN. The reason in Wireshark is. I can ping both sides with no problem and the DHCP option 66 is configured. 

There not much information about this in Google or any other documentation available. I hope anyone can help

Spoiler

NOTIFY sip:MACd476a0f758ce@xx.xx.xx.xx(Phone IP) SIP/2.0
Via: SIP/2.0/UDP xx.xx.xx.xx:5656(FortiVoice IP);branch=z9hG4bKbK4f1731334589;rport
Route: <sip:MACd476a0f758ce@10.100.101.201>
Max-Forwards: 20
Contact: <sip:xx.xx.xx.xx;transport=UDP;handler=dum>
To: <sip:MACd476a0f758ce@xx.xx.xx.xx>;tag=e9b466de-2d30-48e6-80c1-d33fc386c2ad
From: sip:MACd476a0f758ce@224.0.1.75:5060;tag=888888286
Call-ID: 9c0ee096-b8a6-472a-ade1-66fa8cfb4aff
CSeq: 286 NOTIFY
Content-Type: application/url
Subscription-State: terminated;reason=timeout
User-Agent: FortiVoice
Event: ua-profile
Content-Length: 37

[emurray91 · Accepted solution]

It turns out it was an MTU issue. I observer a lot of retransmissions and then lowered the MTU in the tunnel interface and the issue was resolved.