Invalid number error message when sending text from my iPhone

I'm having the same issue on my HTC One M8. I sent a text at 7:10am CST this morning that went through fine. My coworker responded, and when I tried to send another text at 7:12am it immediately bounced back. Since then I have not been able to successfully send a single text message to any of my contacts (I have attempted messaging 4 different people).

Create a new text using the persons phone number (manually entered - not from your contact list) instead of their name. Worked for me.

I am unable to send texts using AT&T Message on my Samsung Galaxy S4 to two of my staff members that have iPhones.  This started yesterday 4/15/15 morning.  It gives the same Error Invalid Number.  Please re-send using a valid 10 digit mobile number or valid short code.  I have tried texting them with a new text string or using an existing one and nothing works.  The number in error also has two 1's added infront of the mobile # as reported by others.  I can text using AT&T Message to others that don't have an iPhone.  So what is up?  Any hope they are going to fix this?  

this didnt work for me either.  went to an AT&T store and they replaced my SIMM card.  still not working.  is AT&T working on a universal fix?  very frustrating to be told to go through steps that don't work.

Charise,

For many of us not so savvy internet/wireless users, when you advise them to "Reset Network Setting" ...it will purge their present wireless modem or transmitter and ask them to have password to get back into that wireless group. Many of us don't usually have the PW to such remembered or possibly traveling and not availabe if written down at home.
Even after doing all of such, my Iphone4S still kept spooling out that error when trying to text message to a contact that I had deleted and all the messages of such deleted. It simply didn't work on my iphone step by step what you said to do. Could be that my contact number was a government phone?

I have this problem too with one phone number. I had done the following:

Removed the contact from the phone. 
Deleted the name and number from recents in texts and the phone book. 

Powered the phone on and off

Reset network settings

Manually entered the number into the text, both with area code and without. 

Removed all group texts including this phone number. 

Deleted a text thread where the number is listed in the text. 

Basically purged the phone of all history or mentions of this phone number.

Absolutley nothing is working. I am so furious and frustrated that I can feel my blood boiling. What the heck At&T??? Get this crap fixed NOW. It is unacceptable!!!!!!! 

Although not yet to the boiling point I'm having these same issues, done all of the above and not fixed yet, thinking a month is a long time for this one issue to have not been fixed.

AT&T do we see a patch or something in the near future?

Same issue with me, but just started affecting only one number starting a couple of weeks ago. They are able to send me a text, but I'm unable to reply or initiate a text. I'm using a iPhone 6+ and have dedicated a significant amount of time searching multiple resources - vendor sites, tech support and message boards looking for known solution for this problem thats reported by a large amount of users and for quite a long time period but to no avail. There has got to be a logical root cause and resolution, but by the amount fustrated individuals, the saga of fail continues.

I've done the following -

-Deleted affected contact from listings, all previous msg history, call logs, and anything else I   could find possibly related

-Reset network settings, restarted

-Sent text to number, trying with\with out area code preceeding

-Recreated contact

-Reset phone to factory defaults

-Updated to lastest iOS v.8.3

No luck...

I even went to the local ATT&T store and showed them. The kid rep tried a few things, but again no luck. Shrugged his shoulders said for me to contact Apple...

This only affects ONE number on the AT&T network. Multiple other numbers on AT&T and multiple other provider networks no problems sending\receiving texts.

me too please help! mine says the same thing the person can text me i try to text back and it says error send with a 10 diget number i do i rest my network also it didnt work!