You could use the iPhone's hot spot and connect an android phone (calyx/graphene) to it and then use a VOIP number on the android and VPN and you would mitigate some tracking but I wouldn't say 'not trackable'. Also, You'd want to turn off cell data (even without a sim) because they still ping the local towers because of E911 requirements
I think Naomi Brockwell has done a video about using VOIP on mobile and how to improve privacy.