It is most likely that you have picked up that Aldi uses the :
More correctly the Mobile Virtual Network Operators (MVNO) buy capacity and coverage on the Telstra network.
It is possible that one phone is connecting to the full Telstra network which is closer (Telstra or full coverage Telstra customer), while your phone is connecting to the further away MVNOs (wholesale) network that Aldi uses.
The difference in coverage can be seen from these two images (source Whistleout):
Full Telstra Network:
Part of the Telstra network used by MVNOs such as Aldi):
There can be differences in reception/coverage not only in regional areas, but in built up areas which can explain why a MVNO phone has different connection strengths/reliabilities to a full Telstra network phone at the same location.
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