In LTE, spec TS 24.301 5.5.1, there are three types of attach request:
attach for EPS only (attach procedure),
attach for EPS and non-EPS (combined attach), or
attach for emergency.
"what will be the respective attach Accept response from the network in successful Case" you can see 24.301 188.8.131.52.4 and 184.108.40.206
You can download spec in here
There is an IE in attach accept sent by network is called "EPS network feature support".
There is one bit called IMS VoPS in "EPS network feature support".
If IMS VoPS is set then network support IMS voice call.
Reference 24.301 220.127.116.11 and 18.104.22.168A
As for Wifi, i think it is impossible to use VoLTE because it is not through the CN of operator, but i am not sure. Maybe other people can help you.