I just tried your suggestion (disabled ALG, forwarded RTP ports and configured global NAT settings). The result was the same, with the addition that the call dropped in about 5 seconds.
Another issue with this approach would be that the public IP of the router is dynamic, not static, but I guess I could use dyndns for that. In my test, I did use the current public IP.
I still haven’t understood, however, why this happens for specific phone numbers.
