I’d be happy to help with this “Invalid Number” error you have been receiving! This may be related to your device storing an incomplete version of one of your contact’s wireless number. For example, this is commonly the wireless number without the area code.
Please test with your most used contacts to determine if you receive this message after texting a specific person. We can then find which contact is causing this. You may be receiving this error message directly into a message thread with a contact, which would help find the source.
Once the affected contact is located, delete all messages to/from this contact, and then delete the contact itself. Restart your device and try sending a new text directly to the cell number before saving as a contact once more.
I hope this helps!
*I am an AT&T employee and the postings on this site are my own and don't necessarily represent AT&T's position, strategies or opinions.