I have an iPod touch, with Ibird, which has a lot of bird songs.
Although they sound very good to me, they don't seem to be loud enough to attract birds to come in. I have tried them in my backyard blind, and even though the birds are right there, they don't pay any attention to the sound.
I think you will need more amplification than your phone can provide.
You need an external remote speaker to make any calling device worthwhile. The bird's can't really hear calls through a phone speaker. A battery powered clamshell speaker (radio shack or the RCA (Better) with a 30-50 ft wire to your device should do the trick. I just bought the RCA and it uses 4 AA's versus 3 in the radio shack version and is louder. http://reviews.cnet.com/portable-spe...-30857096.html
I am using birdjam on iPhone with the $26 clamshell speakers from Radio Shack. Works great.
I agree with the comment about not overplaying the songs, as it can easily stress the birds: play one song (not the whole track) once or twice, and that's usually enough to get their attention.