per plexed

I'm wondering about the difference between "per piacere" and "per favore"? I don't speak Italian often (try close to never) but in calling hotels to speak to my fella, I've had to bust it out. In these exchanges and in my recollection of when I did use Italian more (when I was taking it in college), I it seemed I've been about equally likely to say one as the other, although "per piacere" is the one I tend to stick on the end while if I think I say "favore" at the start of a sentence.

I have no idea why I do this. I think it is an arbitrary positional distinction I have made rather than anything having to do with the language itself.

I am wondering though what is the difference between the two phrases. Does one have a different use than the other or are they more or less interchangeable?