I realize that you started this thread quite a while ago, but I had the same problem, and it's now completely resolved.

Initially, I tried running the Grandstream ATA behind my router, and I had almost continuous problems with call stability. (At least I didn't get any Asterisk calls then, or many others, for that matter!)

I got tired of screwing around with it, and moved the ATA in front of the router. That solved ALL of my stability problems, but then the Asterisk calls began.

I would get as many as three calls in a row, almost always in the middle of the night, with a CID of "asterisk". When I would check the call log, the call wouldn't be listed.

I finally opened a ticket, and tech support explained to me that the calls weren't coming through Voipo's servers - people were actually sweeping random IP addresses looking for unsecured PBXes to make free calls through.

After a couple of tries, Voipo second level support was able to program my ATA so it would respond ONLY to Voipo's servers. It now completely ignores connect attempts from anyone but Voipo. This has worked perfectly for me since then.

I can honestly say that Voipo is now working for me as well as AT&T CallVantage EVER did (and that's saying something).

Gary Sanders