I have a friend with Telus who is the ONLY person I have issues sending a text to.
I have a Motorolla HD Razer.
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Care to explain what happens exactly when you send this person a message? Is there an error message? Are you able to receive messages from them? Do you know what kind of phone they are using? Are you sure it's not an issue on their end if they're the only one you're unable to send messages to?