Grandstream Handsets – Occasional call problems (fragmented SIP header)

The below usually only happens for handsets working outside of the primary LAN but if you are having problems instigating calls (usually internally) then the SIP header could be fragmented and in this case you can turn off the CUSTOM SIP HEADERS to see if this resolves the issue.

When the invite packet is being sent out from the phone, it contains a payload that surpasses the allowed MTU of the router, which once received fragments the Frame into multiple invite that are incomplete and can’t be processed.
In order to fix that, we can whether change the allowed MTU of the routers (might also affect other network devices) in the network, or just disable those custom headers in order to make the frame lighter and make it able to get routed without being fragmented.

Wireshark will show message indicating “Fragmented IP protocol”.

Log into the phone and go to Accounts -> Account 1 -> SIP Settings -> Custom SIP Headings and set ALL of these to No