If the call was already going on in the inner zone and the HO to outer zone is not detected, then the call will still go on, even without BCCH.
MS is not listening to the serving BCCH.
However, Prof, this network topology is very risky. It would be smarter to define your outer zone = 1800 and inner zone = 900. That would let you use the HO algorithms properly. Because in your situation, the in>out and out>in HO algo are certainly tuned in a very weird way 🙂
regards
pix