I just tried this - I had no problems: after 2G Only was selected, I was registered and could make a call in a few seconds
Xperia III on Android 11 as per the standard installation instructions
SIM1: Vodafone UK Contract
SIM2: no SIM (and has never contained a SIM)
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It is probably not relevant, but Vodafone UK were recently saying that they have completed their 3G shutdown
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