I know Mike has a solution with the Juniper device, but for those looking at this thread down the line, I’m adding another endorsement of pfSense. Tons of features, relatively easy to setup, and the price is right. Works great in front of my FreePBX box. Biggest reliability concerns are usually moving parts, so find a machine with no fans necessary, ditch the hard drives, and install on a USB stick.
No best settings for Juniper other than to make sure the SIP ALG is turned off.
Also use the custom service object to make a single object with SIP and RTP ports so you can have one VoIP policy.
For what it’s worth I have used DD-WRT for several years with pretty good results. It works on quite a few appliances too.
I settled on the Juniper, happy!
IpCop works nicely with FreePBX, so I assume pfSense should be same.