As no-one has answered, I thought I would give a couple of tips. firstly, I don’t know the answer, so if anyone does, then feel free to chip in. If I were looking at this, I would start with running some debugs and try to see why the call drops. You don’t say how the router is being used in this scenario, I presume it is a gateway to the PSTN ? Debug the PSTN interface, and also the VoIP functions until you find some messages when it drops. Then you can probably see if there is a parameter that will fix the problem.
Sorry I can’t be more help, it is a bit too long since I did VoIP in anger !