The iTunes Music Store sells its songs in a Protected AAC format that adds restrictions to files which use the otherwise-normal AAC compression format. Unless you’re using an iTunes-compatible phone, you can’t easily play those tracks. There are ways to get around this, but doing any of them could violate the iTunes Music Store Terms of Sale you agreed to by purchasing music. The least objectionable is to burn your Protected AAC files to an Audio CD and then re-import them into unprotected MP3 format. The resulting files will be playable on a non-iTunes phone.