Hi,
Thanks for your replies guys but I'm not having much success I'm afraid, I can't add them to Contacts because there is nothing on the menu to allow me to do so, unlike friends and other contacts where there is. Also, I can't check details either to discover the number to add.
Filip, there is no Unknown sms offer in the Settings > Apps menu but there is an Unknown Source, but that is for Apps from an unknown source.
I very much like the idea that Avast allows you to block calls and Sms but you'd think it would be just as simple to allow specific incoming calls and Sms?
Cheers,
Keith