Another forum contributor pointed me in the right direction.
We need to change the set of phonebooks from “Global” to “Local”. It’s a Qualcomm command (not Quectel) that’s used to do this. So it can apply to other makes of modem using a Qualcomm chipset.
Switch to local phonebooks: AT$QCPBMPREF=1
Switch to global phonebooks: AT$QCPBMPREF=0
See forum article: EC21-EUX read MSISDN