you are right, jio4gvoice needs data on to work where as volte is not data dependant to work. in fact in my experience its smoother to call on volte.
lot of times other network numbers dont connect in one go, but if you keep auto redial on (if your phone supports) then in within seconds it tries multiple times and the success rate of call connecting increases. no issues on calling other rcom or jio numbers, they connect almost instantly.