I have a Xiomi Redmi 9C phone. In the mobile network settings it was set to prefer LTE which I took to mean it supported 4G

I did call the Spark help line a few weeks ago to check and they said it should work but maybe a firmware update would be a good idea. I didnt do this

Anyway today it looks like voice calls are not working. A bit difficult to check but looks like when people call me it goes straight to whatsapp (wifi?) calling. Have to try with someone that does not have whatsapp... However it does appear that texts are going through - I got a txt from my partner and replied to it and she got the reply but if she calls me it goes straight to leave a message. If I try to call the mailbox it dials it but nothing happens

Bit confused if I can get txts why cant I get calls as both must be using 4G?