Getting music from old IPhone into iTunes.

Hello, after one of the last iOS updates, I noticed over 2000 none-purchased songs are missing from my music app. I went into iTunes and accidentally synced my phone, so now my music is also gone from my iTunes library. I found my previous iPhone in a drawer which still has all my old music. How do I get the music from the old phone into my iTunes library?

Thank you all for your help.