At one time, in v11, you could create an extension and set it to forward to an SPx. They were in the drop down list.
So then you would set the extension to away and whenever someone dialed it - it would forward to the SP and grab the call.
It wasn't perfect... but it worked.
So, someone on a remote side, could dial, for example 6001 and your outbound rule of 4 digits would go to remnote bridge and be forwarded to SP1 on the remote side.
But in one of the service packs, the ability to forward an extension to an SP was removed.