I'm not sure if this will help in your situation or not, but this works for us. In SRST 3.0 what we did was have the "default destination" and main number mapped as the first line appearance on the receptionist phone. Used "call-foward busy" to map to the second line appearance on the receptionist phone. This will roll any inbound calls to the second line if the first line in use. Then used "call-forward noans" to the third line appearance on the phone. If the receptionist transfers the call and it rings back, it doesn't occupy any of the first 2 lines. Finally we gave the receptionist a 4th line appearance for making any outbound calls without affecting the inbound lines. Then we set the "limit-dn 7960 4" so she'll have all 4 lines in SRST. It works great for us
Good luck