Has anyone here configured sip phones for use on Officeserv systems much?

I am having an issue with the in/out group function with 4 digit station groups. I'm trying to use the 53+staion group#+0 to log out of the group. Works fine from a digital, IP, or SLT, but for some reason the sip phone won't do it.
It works with 3 digit station groups.

I though it might be because it results in a 7 digit string, so I tried changing the feature access code to *53 and had the same effect. I also tried changing it to just one digit. Same result. tried changing the station group to 6000, again same result.

Very wierd.