If you are using the embedded MBG on the MiCollab server, it is very easy to see if you have a problem between the MBG and the MCD. Just do a packet capture under the Administration-Diagnostics link. SIP signaling between the MBG and the MCD will not be encrypted so you should be able to clearly see why the Registration failed. If you don't even see a Registration attempt, your client is not able to reach the MBG.
Also, watch out for lockout periods on the MCD. If you make a mistake on the SIP password, you will be locked out for 15 minutes. A log will be generated on the MCD which will clearly tell you this. I have wasted too much time in the past with this not realizing I had a wrong password and no matter how many times I reset it, the account stayed locked until the timer expired.