Why is iPhone adding a '1' to my phone numbers when sending a text?

I sent a text message to (123) 456-7890 and an error message immediately appeared to "use a valid 10-digit phone number" and showed the number I used with a 1 before the area code. I tried deleting the contact's number and re-entering it, but the same thing happened. Why is the phone adding a 1? I've searched through settings (and I did turn off international dialing help to no avail). Is this a setting issue or...?

Solution 1:

The US telephone system use a #1 in front of USA area codes.

If you with in the same area then you do not need to dial it.

Even if you dial '1 in local calls it will be just ignored.

You phone automatically adds a #1 anyway so you do not forget it.

However if you are outside the area then #1 is needed in front of the number.

In modern days devices the #1 is hidden but it is dialed automatically.

Solution 2:

I was just able to duplicate this behavior sending a text to a number not in service.

The exact error message arrives as a text message from a network number and reads "1XXXYYYZZZZ Error Invalid Number. Please Re-send using a valid 10 digit mobile number or short code." (where XXXYYYZZZZ is the bad phone number).

It means either you are texting a number that is not a mobile, or "I'm sorry the number you have texted is not in service"