Hello @Piotr Sobczak
Thanks for reaching out to us here on the community.
We are very sorry you initially had some issues with the incorrect number showing when messaging.
Just to confirm, are all your messages now displaying the correct number?
For others who may be experiencing similar issues, we would recommend taking the following steps.
1) Open Messages, go to Settings (top right, Message Settings), open "Chat Features" or "RCS Chats", verify if this is the correct number or not.
2) If it shows the incorrect one, then head to: https://messages.google.com/disable-chat and you should be able to unregister the number. Once done, turn off the Chat Features/RCS and the phone. Wait a few minutes and turn the device back on and re-enable Chat Features/RCS it should let you re-register/update to the correct number.
Alternatively, you can turn Chat Features/RCS off and test SMS after this.
Please let us know if you require further support.
Thanks,
Nat