Avaya IP Phone IPSec - Wrong MTU going out WAN

[burberrybulgarian]

Greetings OPNSense forums. I'm currently working on moving off of the terrible *fsense and over to OPNSense. I'm currently running into one weird issue with a Avaya IP Phone that uses IPSec to connect back to the office.

It looks like the tunnel going out is getting a MTU of 1242 for no apparent reason and is fragmented. This is going out of the WAN interface. The tunnel is able to push 1480 MTU.

I've tried everything in https://forum.opnsense.org/index.php?topic=20412.msg99133#msg99133. Setting the WAN MTUto 1250 does indeed fix it but this is a dirty hack and slows down a gig connection tremendously.

Anyone else have any ideas?