If the message is blue,(iMessage) then possibly that Android number was previously registered as iMessage on iPhone, and it needs to be manually deactivated by the other user.
If the message is green (text message), it is a failed text message. I suggest calling Telus and seeing if they need to update the software on your SIM card, this just happened to me.
I am going to say this is a Telus issue. I went back and forth with Telus and Verizon over a couple of months because I could send texts to my friend on Verizon, which she received, but I was not receiving her replies. Even sitting in the same room would not get the texts she sent. We tried different text apps and yet I wouldn't receive them. Verizon confirmed the texts being sent but once they leave their network, they obviously have no control over them. Telus would say, nothing was wrong but it ironically was fixed after Telus support looking into it and the ticket being closed with no explanation.