How can i be sure that my friend is online if he's using yahoo messenger on his mobile phone?

I have a friend in my yahoo messenger list that has status "I'm Mobile" with weeks. I send him msgs but I don't know if he receives them. Sometimes he goes offline and signs in right away. Do you know what does it mean? And is he really online?
Update: when i look in my address book it says that he's offline. but in my messenger it shows that his status is "I'm Mobile"
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