Periodically Switching down to 3G.

Hi All! I recently called one of my spare numbers on purpose which I knew was switched off. But I noticed, the signal bars suddenly changed, so I enabled data to see what was happening, it turns out when your calling a switched off or invalid number, iD for some peculiar reason drops the call down to the 3G network instead of staying on 4G/5G. I have completed the same test on iD’s host network, Three and it doesn’t drop down to 3G when an invalid number is dialled. Anyone know why iD does that? SMARTY also doesn’t drop down to 3G.