If you want to manage sip.conf in salt, it needs to contain passwords, which are therefore visible to all minions. Even SIP trunk password must be visible. Can't have that...
I just extended this formula to pull this private data from pillar, should I prepare a pull request? I actually did more (generating a simple dialplan based on users/extensions/dids), but it needs cleaning up first. Moving passwords to pillar is the most important thing to do IMO.