How much were you charged for this call?
Did you place the FaceTime audio call by phone number or email address?
I think this depends on how the FaceTime audio call was placed whether using a phone number or an email address. If using a phone number, it seems that when an iPhone loses Wi-Fi connectivity, the call falls back to the cellular network to maintain the audio call connection.
One option is to place FaceTime audio calls using an email address to prevent your iPhone from inadvertently accessing the cellular network.
Another option is to turn off Wi-Fi Calling, enable Airplane mode, then turn Wi-Fi on. If you’re not subscribed to Wi-Fi Calling, enable Airplane mode then turn on Wi-Fi. This should prevent your iPhone from accessing the cellular network if or when your iPhone loses Wi-Fi connectivity.
Unfortunately, Wi-Fi is not foolproof. The number of devices connected to your Wi-Fi network, Wi-Fi channel interference and whether using 2.4GHz or 5 GHz Wi-Fi, can have an effect on the quality of the Wi-Fi signal.
The link below sounds similar to your situation.
Charged for FaceTime audio call
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