This looks like a Samsung problem, not an AT&T problem. Like Robert Said, someone reported an issue with T-Mobile. I'm having the same thing, this morning, from multiple contacts, with an AT&T Note3
S4 on AT&T. Same thing started happening to me in the middle of a conversation at 4 this morning. Out of the 6 people that have sent me texts, 2 of them are normal. The 2 people I can talk to normally have AT&T, but there's 2 with Verizon, 1 with Virgin Mobile, and 1 with AT&T that have that extra 1 in front of their numbers. Extremely frustrating...
It's AT&T, nobody I work with is having any problems with their samsung phones except the one person with AT&T. T-Mobile might just be a part of it as well. No problems with Verizon or Sprint users.
It has to be a network issue. When receiving texts from other At&t users, I don't have a problem. But I've had the issue with 2 contacts this morning-1 is on Verizon and the other T-mobile.
This is happening to me as well, but to only one of my contacts I've reset my phone. Deleted the contact. I'm not sure what else to do to fix the issue 0_0 but it's driving me crazy HELPPP