[Serusers] Question about PSTN Gateways and SER.

Hello List. I'am starting to use SER as my SIP platform and i'am still in a testing phase. I would like to ask you a question. If we have several PSTN Gateways connected to SER, we want to be able to select one of them depending on the destination number dialed, so far this is no a problem, we use a combination of "if" with the "URI" and a rewritehostport() and works ok. But now, the gateway termination is needed to be dynamic, so if we have any problems with a destination in one of our gateway we want to route to another gateway, maybe changing the "rewritehostport("IPGW2")" to another gateway. The problem with this if we need to restart the SER service every time we make a change in the ser.cfg file. So here is my question. Is there another way to do this?, maybe a Failover PSTN Gateway Selection, i heard about a B2BUA server can do this task. I rellay hope that someone could give me a clue about this matter. Thanks in advance Best Regards Ricardo Martinez.