If what you are getting is a disconnect when the call gets transfered out at the far end (i.e.: the recipient end, be it an auto attendant or a person hits transfer and sends the calls over a different circuit, usually a Pri), then, what you most probably have is a protocol mismatch. You might be on 5ess and the CO is on NI-2, or you are SL1 and they are 5ess.
Another viable explanation for the same case (disconnect on transfer) is that the CO is marking your calls as private (it is possible on ISDN), therefore, when a transfer out happens, the privacy marker forces the switch to drop the calls. This is true sometimes on calls from Federal entities, which lately looks like have implemented privacy. I already hit a case when calls from them could not be transfered out or conferenced, no matter what, but if I called from my cell phone, no problem whatsoever.
So far I'm just rambling on the possible scenarios. You need to gather a lot more information on the particular situations when the calls get dropped. I suggest you try to replicate the problem under controlled circumstances, where you make the call and you also receive the call and transfer it out, maybe another circuit at your own place, or even start by calling your own Pri and transfer out.
And, you can also request your LEC or CLEC to monitor one of those calls and figure out the reason for the disconnect.