I often see my cousin status to be "I'm mobile" but when chatting with him, he recieves no message from me. He doesn't know how to turn that feature off. Please guide us how to.
That's because he has registered his phone to use Y! Messenger via texts. And the status appears because he has set Messenger to sign in automatically to the phone when he signs out from the desktop Messenger.
Look in the Preferences to turn that off.
Also, when you send a message, he should receive a text. Try updating the phone number in the settings to use the feature.
I think , he has registered his mobile number in Yahoo messenger.
Resource from here