So as the title say I am unable to send texts to one person. I can call them perfectly fine and the texts register as sent, but the person never gets the texts. I receive their texts and calls as well. I had this problem on my last phone, a Samsung Galaxy S2 (Model: SGH-i757M), and still have it on my new phone, an iPhone 5s (Model: A1533). This problem only happens with one person.
I can imagine how frustrating it must be to be unable to correspond through text messaging with a specific number. We look forward in helping you resolve this issue.
Do you know if it was a number you previously were corresponding with through iMessage before they/you changed phone (or vice versa)? What type of phone are they currently using?
It would be very useful to get some more details on the troubleshooting steps you've gone through so far, as well as any error message you may have seen.
Do you know if they may have blocked SMS from your number at a certain point and forgot that it had been done? You may want to ask the recipient to check. I know it may sound silly but it happens more commonly than you'd think.