Cannot activate WhatsApp

[#1]

Hi, first post here. Installed a Plusnet SIM on my wife's Galaxy S10 yesterday. All good except WhatsApp. Cannot activate a WhatsApp account for this number as the SMS with the 6 digit verification code never arrives. The "call me" option doesn't work either. WhatsApp verification screen displays a message containing "we couldn't send an SMS...". Why is that? So I took the SIM out and put it into an old A8. Same issue. Both phones have previously run WhatsApp.
Clues gratefully received.

[#2]

You most likely need to request the 'adult content' lock on your SIM to be removed - call 500 from your mobile.

[#3]

@jab1 - not sure it's related to the content filter as it looks to be set correctly in this case.