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