I've got a number of voip setups using voip.ms as the DID provider.
I use pfSense as the central router and I've never had to "register" the router, just the end device(s).
In most cases I setup an Asterisk box to handle local phones, but I have registered phones directly.
In many cases, pfSense has not needed any special configurations at all, others required a few NAT tweaks depending on the ISP at the local end.