Well, if you're going to make them untraceable, you have better had already figured out from who you're making them untraceable to.

If from the telecommunication companies, you're pretty safe. But from governments handing over subpoenas to the telecoms for data? Less so. Don't make your service another Encrochat, have a TOS that prohibits and bans illegal use of the service (this can be accomplished by keeping temporary logs of the phone calls) so that you stay on the good side of the law.