Sim has changed accounts

[Dizzaloo]

I have an O2 account which just has my sons phone on now as I stupidly moved my contract to EE last year. Son has/had an iPhone 12 but it’s broken so he put his sim into his Samsung S21…as soon as he switched on the S21 with his sim in it has swapped to my old account. It has deleted his contacts and given him a random handful of my contacts. He can no longer make calls, send texts or use his data but when I ring his number it works. When he puts the sim into an old iPhone it works as well. 
it’s really weird…does anyone know why it’s doing this and how to enable him to use the sim in his S21?

thanks

[MI5 · Accepted solution]

@Dizzaloo 

He needs to do a factory reset on the Samsung.