We are having an issue with some VoIP phones in one of our offices, they periodically fail to register, we can still ping them internally but cannot make calls etc. Forcing them onto a new IP reservation then rebooting fixes this. We have run DHCP from the router, server & set them IP's statically but seemingly makes no difference.
On running a few packet captures from the MX84 on the LAN interface, I can see traffic between one of the non-functional phones and the external IP of the VoIP provider, including some outgoing only SIP packets-no responses coming back in. However, on the WAN side capture I cannot see these outgoing SIP packets (nor any incoming) for that internal IP address.
On assigning a new IP and rebooting the phone and repeating the captures, SIP packets for that phone are showing on both the WAN and LAN interfaces as expected and the phone works.
The phone provider suspects a NAT issue, we have no 1:1 or 1:Many NAT mappings set up. It seems as if the outgoing SIP packets are reaching the router and not being forwarded out over the internet, however the phone can successfully make TCP connections. Also this issue is only periodic, and affects all phones which have to be manually assigned a new IP to get them working again.
Leaving me a bit baffled as to what is causing this and why the SIP packets are not exiting the router (or at least why I cannot see them). Any help would be much appreciated on this.