AT&T Cell phone customers cannot make calls to my VOIP (Nextiva) Business phone

As of this morning we are unable to receive any calls to our business phone line from customers calling in on AT & T cell phones.  At & T landlines are fine as well as all other cell phone provider.  Nextiva attempted to troubleshoot on their end and it is not on their end. 

Please advise what the issue is and how to resolve.  Thank you