Wifi Calling and Asterisk SIP Account I would like to know if its possible to set up a generic sip account under "wifi calling" or Android sip accounts to use your phone as an extension with, say, Asterisk? 

I can get the phone to register with Asterisk and when asterisk attempts to bridge a call to my SIP account I can see in the captured SIP packets, that the SIP packets are reaching the phone and being responded to. The packets returned from the phone are ringing sip notifications however the phone does not ring.

I also find it impossible to tell Android to use the SIP account for outgoing calls. I enable, "for all  calls" under SIP accounts but it still tries to dial over the SIM.

I can get everything working with a SIP client like csipsimple. It looks like the "SIP account" support built into asterisk is only for use with a cellular service provider that provides a SIP account in addition to their normal 3g/lte connection?

Is this true? I seem to remember earlier versions of asterisk supporting SIP accounts for PBX's like Asterisk in the early days without the need  for 3rd party apps?
yes it is possible to setup astrisk sip account and use it to dail out from android standard dailer also you can receive calls with it, but as i know you can not setup a straight forward wifi calling because it is not a standard sip protocol and it is a carrier specific service integrated with 4G.
You can emulated such service with alot of work on PBX server side and the cliend side. Finally these is nothing impossible.