SS,
Sorry, I haven't run into that before, but let's see what we do know and what may have been missed.
The 6900 has to be able to communicate with the MiVO-250 via SIP [5060] and to the MCS via other ports [69, 80 and 8202].
So, it sounds like you are getting to the MCS, but not to the MiVO-250 since you should still be able to use the phone if the MCS is down.
Did you make sure to put the Public IP Addresses in the locations you need it for SIP phones to work; Under IP Settings and IP Configurations?
I would look at the licensing for both the MiVO-250 and the MCS since the phone has to have licensing on both servers.
Thanks,
TE