VoWifi is actually a "paid" service from a Telco. It is not common in SG. Starhub actually supports it but only on selected devices. Don't think it as VoWifi is a free service. Cause this is still ride on mobile network over using wifi.
Not sure that it is a paid service or not. All iphone and samsung phone comes with VoLTE and VoWifi enabled. Even an old Samsung Note 8 has it enabled on both Singtel and Starhub network
My wife's Poco F3 comes with VoLTE and VoWifi enabled too on Starhub network.
My Oneplus 7 Pro after the hack VoLTE and VoWifi are enabled on Starhub, Singtel and M1 network.
I tot it is by default to hv both enabled on 4G network which is an old network that will be replaced by 5G in years to come. 3G might be reaching EOL in few years time.