unable to sent text message to specific contacts

I have a reoccurring issue with my iPhone 6 since I purchased it in April. It will not send an SMS or an iMessage to a specific contact. I can ring the contact, receive a call and receive a text, however when I try to send them a message I receive a standard "Not Delivered" error. It isn't just one person either, there have been 5 or 6 different contacts with the same identical issue. I am running up to date software, 9.2.1. I have reset my phone to factory settings and restored it with no change. Turning off iMessage does not fix the issue. A friend of mine with an iPhone 6 also has the same identical problem. Has anyone experienced similar problems? Any known solution? I have tried googling the issue since I first got the phone but haven't been able to find anything helpful