for 1800band you should set RXLEVAMI=15 (or12). And also to set RXLEVMIN (for handover). I thing that it is the reason, why you could not setup call and your mobile missed paging.
Your problem probably is, that you don’t have Path balance (Uplink-Downlink). Your mobile could listen BCCH, but BTS don’t.
It is wrong to set same RXLEVAMI for both band, if you have sufficient capacity in 900 layer.