Hi MKT,
From what I understand, you have your BCCH on the 900 band, and TCH on the 1800. Try to change the values of parameters which control the triggering of the call to/from 1800 band after the call is established on 900. In Nokia, the parameter names are NonBCCHLayer Offset (usual value 10 dBm), nonBCCHLayer Access Threshold (usual value -90 dBm) and nonBCCH Exit Threshold (usual value -95 dbm)