Have had my phone repaired twice and after both fixes it will still not send a sms message to ant O2 contact that I have, all messages come back "Not Delivered". When is type in chat bot the send arrow stay blue whereas when I try to send to EE numbers It turns green and send straight away, I can receive from O2 numbers, the sim card is new are worked perfectly in my Huawei phone while repairs where taking place.
The number is my other number so I know its correct, and can receive from it but not send back to it, both the same phone and have same setting for both, If I use my Huawei p20 it work fine both ways
Can you check Amy relevant text blocking facility in the phone just to ensure the number hasn't been added to it accidentally.
If not then delete all text and call history for the number and then delete the contact from your Contacts App and them soft reboot by turning your phone Off and On.
Then re enter the contact and see what happens @Layby
Current Phone > Samsung Z Fold³ 5G.