I can easily send texts to most people. However, two numbers in particular always return a "Error Invalid number. Please re-send using a valit 10 digit mobile number or short code."

Now, these are two of my friends' numbers. They also have iPhones and cannot send me a text either (they get the same message). We can call each other just fine though.

We've tried to text each other's numbers using 7 digit, 9 digit (area code) and 10 digit (1+area code) to no avail. Always the error.

How do I fix this?
Yes, thanks to the first responder, my numbers here are off, but still, we've tried each others' numbers with the 1+area code, the area code and the number, and just the number. I did my math wrong with the 10, 9, and 7 above. But the question still remains as the correct numbers still don't work.