Sos in corner of phone
Upgraded phone for gift now old phone has sos in top corner. Not giving new phone till Christmas
iPhone 8
Question marked as
Try to do a force restart -> Force restart iPhone – Apple Support (UK)
If it doesn't help, remove and reinsert the SIM card.
If it doesn't help contact your carrier, maybe the SIM card is damaged, ask for a replaced (new) card. Normally it is free.
Ask to check if the SIM tray is bent, as even a tiny misalignment can cause the SIM card to not fully connect with the internal contacts in your iPhone.
If it doesn't help, see these Apple articles below:
If you see No Service, Searching, or SOS on your iPhone or iPad
If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support
Manually update your carrier settings on your iPhone or iPad
Manually update your carrier settings on your iPhone or iPad - Apple Support
Update your network provider settings manually on your iPhone or iPad
Update your network provider settings manually on your iPhone or iPad – Apple Support (UK)