[Serusers] LCR outbound and inbound

All, I was thinking of using LCR outbound and inbound for a couple reasons. Inbound for the sake of some simple redundancy (and load balancing) and outbound for the sake of LCR (cheapest provider). The problem I'm facing is LCR feeds off of one table, while outbound is rather trivial I was wondering how I would work with inbound. I'm assuming I could put 10 digit equivelent users phone number in the LCR table. However, if it did *not* exist in the table it would potentially send it right back out the to the PSTN which could cause a really nasty loop. Has anyone done this? I have a couple thoughts on how to get around this but they all seem too complex and/or hardcoded. I'm trying to avoid external scripts also, if possible. We've had bad luck with those :-)... Matt