Depends on the request, how frequent the calls are and how long the presumably one-sided relationship has gone on and, generally, how much I am actually put out in helping another.
If I can help, I help.
However, if I've helped and the other hasn't returned the 'favor' (not a 'me this time, you next time' tradeoff, but generally speaking, you know, if it's <always> one way and only one way), then at some point, it increasingly depends on what's being asked and the things I noted above. It's great to always 'be there' for others, and that's pretty much my default behavior; however, I've learned that one can be doing a disservice to oneself. Strangely, perhaps, one gets into 'taken for granted' territory and the reality that one person is doing 'more' sometimes gets lost. That is, helping for the sake of helping is its own reward, but there is a line that, if crossed by another, and left unrecognized, detracts from one's own life.