VoLTE doesnt count against your data plan, you dont even need a data plan to use VoLTE. In fact, most carriers require VoLTE these days so when you first insert the sim card, your carrier has sent a configuration profile instructing your phone to turn it on. You can turn the data to “off” but it still leave it on for VoLTE calls only.
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You mean like a VOIP number like Google Voice? Then no, your call get dropped when you move out of wifi range.