Conditional execution based on input string's prefix
I don't like too much the fact that you use a String as the returned type of your function.

What about using a class hierarchy or exceptions?

I see opportunities to create a hierarchy even for the messages. Why don't you create a BotMessage and a SkypeMessage?

Both should have the logic to decide what to do in the base case, maybe in an analyze method. The analyze method of the SkypeMessage should, in addition, manage the Skype-specific aspects.

Finally you should simplify the code and remove the list of if statements.
Introduce a new set of objects to parse the messages. These objects should have a match method that check whether the string matches with their definition, and a performAction method that generates the return value.