Buffered outgoing text not delivered

phone texts I’m sending from an iphone8 on O2 network to a non smart phone (Samsung GT3590 flip) on tesco mobile are no longer being received although I can call the phone and Receive texts from it. All other contacts are fine on both phones. Have tried deleting and reinstating numbers from both phones, switching both phones off and on, checked for number block/reject, removed sim from Samsung. If I send the message prefaced *0# the track message says Message for , with identification has been buffered. Any suggestions what else I can try?

[Edited by Moderator]