Woolworths uses the Telstra wholesale network, not the full Telstra network. There is definitely a difference between the two, more so in regional areas, with the wholesale network coverage being (essentially) equivalent to that of Optus.
Coverage differences aside, your Nexus may not may not cover the bands required for Telstra 4G, as you mentioned, which would also affect coverage. From memory Telstra are now using band 28. But if it is happening on the iPhone too, well that would negate that idea as it covers band 28.
And with dropping back to 3G, Woolworths may not allow VoLTE, being calls over 4G. So as soon as you make a call the phone will switch back to 3G, with 4G only being used for data.
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