My old phone sometimes relieves texts sent to my new, how do I stop it?

[AC Question]

I have my old phone (Galaxy s5) receiving random texts from a friend that has sent that text to my new phone's number. It does it randomly and I cannot figure out why and how to make it stop. My new phone is a galaxy s7 edge. They have completely different numbers. Please help.

 

[Rukbat]

Who's the carrier? The reason varies depending on them.