a) that person is wondering whether you're active/ available to answer a query or something like that immediately or
b) it implies, "Yohooo... anyone there? need someone to talk to or
c) Call me, like now? or on a lighter note,
d) it could be something positive especially when it's followed by a smiley :)
I don't know with you but if I couldn't seem to find the right words to say and I badly needed someone who'd help me understand what I'm going through, I text my closest friends with "friend?", they'd answer back and the next thing you'll know, you can't stop blabbing about the thing that bothers you and in no time you'd feel some sort of relief...
That's why I love my friends so much. They know me when I don't feel okay or am I just a give away? Either way, I love them.