^
like I said earlier, maybe its your phone activation and settings that are not properly updated/configured/activated and not a hardware/bug problem that you are stating....
I am using with reliance, with data full on and even then I rarely miss a call (even though I should, as data and voice dont travel simultaneously on 1x), though sometimes when my phone is downloading something my callers do get a busy tone but never switched off. So there is NO SUCH BUG in the phone or hardware. I would suggest you stick to 1.56 based roms as of now (modaco 1.3 is most stable), in which the only issue I am facing is of BT file transfer and nothing else. I am waiting for the official 2.1 release by sprint after which I will shift to that...
kindly try reactivating your phone, if its possible or take the help of your seller (sadikk) and manually configure tata settings once more... be aware that tata operates on multiple sid's and nid's unlike reliance which has only 1; so get all those feeded into your phone through qpst...