It's hard to diagnose any calling issue without the actual log or pcap data of that call example to review. Given your one-liner paste it looks like the message is malformed.
This might be a case of transport changes between UDP to TCP or vice-versa, or that your UA is exceeding 1500 in the UDP message. There are lots of possible causes and scenarios.
I'd recommend looking at the registration of the phone in the webui (users - {user} - registrations). If you see in the Contact: of the registration a flag "transport=tcp", then the phone is registering via TCP which does not have a size limitation.
If you don't see the "transport=tcp" in the registration Contact, then it's registered via UDP which has size limitations.
To determine if the phone is exceeding the UDP size limitation, packet capture the phone as you reproduce the scenario, then look at the size of the SIP messages it is sending. Over 1500 is bad. Another way to investigate transport changes is to look at the Via headers. If you see UDP, TCP, UDP etc. stacking there, then this is likely occurring.
Often you can reduce the size of UDP messages a phone is sending by removing unnecessary codecs or injected headers from the phone configuration.
PCMU (G.711 U-law -- PSTN standard in North America), and PCMA (G.711 A-law -- PSTN standard in parts of Europe/Asia) are required. Everything else can be removed safely and can't be used on the PSTN anyway. G.729 is the exception but must be supported by your provider and is usually transcoded to PCMU/PCMA somewhere along the line anyway. It's more headache than useful IMO.
I've also seen these issues caused by the admin neglecting to configure the correct intranet subnets in system - settings - internet calling. You should only have local networks listed there and everything else removed (include your VPN networks if any). If you're missing a network, and the phone source IP is within that same missing network, the SIP traffic from the phone will run through NAT traversal functions rather than sending messages directly to the phone.
Good luck,
Matt