Currently we are using Topex, voice quality is very good (not too much feature but it works fine), also there is 2N which has many features (problem is price), we tried with dinstar also but voice quality was awful, we put another dinstar because we thought that there is some hardware issue but we had the same problem (except that routing features are nice). There is one more gsm gateway that we tested, Portech, sound quality not so good too...