Wifi Calling and Asterisk SIP Account
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.