when i call my 4g turn to 3g

Your phone will use 4G/LTE if it is available. It will use analog voice otherwise. Voice is called GSM; there is no 3G/4G, which only applies to data. If it switches to 3G that means that the carrier's network does not have sufficient capacity for VoLTE.

It sounds like your carrier can do voice+data, but it's still a network problem. Have you tried updating carrier settings? See this Apple support article-->Update your carrier settings on your iPhone or iPad - Apple Support

This is a matter for your carrier

Some carrier’s networks cannot do simultaneous voice and data.