welcome to the Polycom Community.
Such information can be found in the Admin Guide as an example UC Software 5.5.0 on page 338
The default is 1496 but it can be changed from 800-1500
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Polycom Global Services
Thank you. We are trying to see if changing the MTU size on the phone itself will help an issue we are seeing. We have about 5-6 connected via a VPN tunnel and we cannot get them to pull down any changes that are pushed to them over this tunnel. If they are in house (in the office) they are updating as expected. The network engineer thinks if we can adjust these 5 phone's MTU size to 1360 that we can stop this behavior.