Problem with Yealink

[kmageean]

I have a licensed PBX but can only register 2 phones at a time. Is there any reason for this. We have checked the router settings and can not find an issue. The PBX is external to the Lan. It will only register two devices and sometime 3 at one time. The phones are Yealink T20's. I have used these in a number of locations and this is the first time that I have experienced this issue.