First off, your router should resolve this situation gracefully. It's called "glare" and the router and telco switch should mutually figure out that they're trying to use the same channel, and abort or move calls accordingly. You should not permanently lock up DSPs. This is a bug and you should contact TAC about it.
That said, it's probably a good idea to start at the top of your trunk group when dialing out. I believe the "isdn bchan-number-order descending" command will do what you want. You apply this on your single NFAS Serial interface and it should start at the top of the group as a whole.
http://www.cisco.com/univercd/cc/td/doc/product/software/ios113ed/113aa/113aa_3/natisdn.htm#xtocid1945315
Correction: I did some more research and this seems to be the default, and I'm not 100% sure it will have the effect you're looking for on NFAS, which is to say it will try the "last" trunk first. Try it anyway, though. There is a series of "trunk-group" commands that allow close control of what channels to use, but that is only available on AS5[38]XX. Also, please do open a TAC case for the DSP lockup problem.