Little question regarding mobility on CUCM7.

A group of people needs to change the destination of a Mobility Remote destination based on their availability.

the problem right now is when I am trying to reach that number it get busy if the number dial in the destination profile is an internal extention, it only work when it's an PSTN number...

I guess there is maybe something to play within Service parameters : " Inbound Calling Search Space for Remote Destination  Required Field   " and CSS but it did'nt work on my side..

any idea ?