Dear Philippe Lindheimer,
Well, the attitude of Deutsche Telekom AG reflects that they are the incumbent and still dominant carrier in Germany operating hundreds of VoIP servers. While they push for fast migration to VoIP in order to scrap ISDN, they do not care a bit about users of Asterisk and the like. They rather want to sell their proprietary Speedport routers most of which are far too limited in quality and functionality to be used in any (semi-)professional environment. There is technical documentation about Telekom's VoIP functionality available, hundreds of pages of it, but rather illegible and out of touch with reality, as it tends to be outdated, different servers seem to have different policies at each point in time and issues such as load balancing and proprietary DNS resolution strategies remain behind the curtain. Smaller competitors have working sample Asterisk configurations available for their customers, of course.
In the past, I did only open and forward a small range of ports (30000 - 30099), which are a subset of the range claimed by the carrier. I did also limit that to RTP traffic coming from Deutsche Telekom's servers. I do hesitate, however, to open and forward ranges very broad ranges, such as 10000 - 20000. Furthermore, limiting port forwarding to traffic originating from the carrier's addresses is also tricky, as they refuse to reveal from which IP range their traffic is sent, so it is a constant game of watching and updating. Thus, I intend to avoid RTP port forwarding as long as I can while watching for adverse consequences. I did try this last about 18 months ago and as you said, it did work for some time and then it did suddenly stop.
Regards,
Michael