Basically, when the first 4G and 4G LTE networks were first being rolled out, the technology to make voice calls over 4G networks wasn't quite ready yet. So the earliest 4G phones dropped back to 3G for calls, but used faster 4G networks for internet and data.
Eventually, the telcos rolled out a technology upgrade that allowed calls to be made on 4G LTE mobile networks, called VoLTE (voice over LTE).
But it only worked on newer 4G/4G LTE phones that supported VoLTE for calls. Older models still dropped back to 3G for voice calls
Sounds to me like your MIL has an older 4G LTE phone that still falls back to 3G for calls, instead of making them over the 4G LTE network.
If that's the case, she'll no longer be able to make calls with that phone once Optus 3G shuts down.