Ive got a Yealink VOIP system setup. The Yealink is connected direct to one of the LAN ports on my Draytek 2860 router (FW 3.8.9.5_BT) The system works fine if i dont have SRTP (call encryption) enabled. But once enabled the VOIP calls stop working. Ive done some troubleshooting with Yealink and they have advised : "The INIVTE packet is too huge, it require fragment. Why the first INVITE can send to server and get 407, because this first INVITE packet doesn't contain some authentication, the frame is smaller." And that i "need to enable fragmentation on the router."

I have my MTU set the 1492. The max suggested by my ISP.

Ive already checked under Firewall > General Setup and "Always pass inbound fragmented large packets (required for certain games and streaming)" is enabled.

Any idea how i can enable fragmentation the required on the 2860 ?

(Not sure if its relevnat but at the moment im using UDP for the transport setting for the VOIP system, but im soon to change this to TLS. Not sure if thats relevant for the above)