Certain people are not receiving my text?

[AC Question]

I have a samsung galaxy s 5 with T-Mobile carrier. Ive had it for about 14 months now and it has been working fine in terms of texting. But starting about a week ago a friend of mine with an iphone stopped receiving my text. I will still get messages from her but she wont get my text at all. I cleared data and ive restarted my phone. Im usually always on 4g but it does not appear to work on wifi either. Any help would be greatly appreciated.