I was wondering if there is something I can adjust for more line quality. I don’t think it is the DSL connection, because if I use another provider directly from my phone without going through the Freepbx the quality is fine. Perhaps there are some pointers regarding this.
You say absolutely nothing about your setup for anyone to have the remotest guess as to what you need to check. Most people have no such problems using “standard procedures” on well controlled hardware.