If you bought them recently they are presumably not suffering from DRM, which has not been applied to purchases for some time. (You can tell because they will be described as 'Protected AAC' if DRM'd, or 'Purchased AAC' if not.) It's more likely that your phone simply won't play M4a (AAC) files. If they are not DRM'd you would need to convert the files to MP3 following this procedure:
Firstly, go to iTunes Preferences>General and click the 'Import Settings' button. Set 'Import Using' to 'MP3 Encoder' and use the next drop-down menu to choose your settings - you can choose 'Custom' and set the bitrate as suits you.
Click OK, then OK again to close the Preferences.
Go to the Music list; select the files you want to convert and from the 'File' menu choose 'Create new version' then 'Create MP3 version' (iTunes 11 - in earlier versions choose 'Create MP3 version' from the 'Advanced' menu).
MP3 versions will be created and placed in the music list (the AAC versions will remain there). You can drag them out if you want to save them elsewhere (doing this will not remove them from the Library). You can select and delete any files as required in the Music list. (Never tamper with the actual library in the Finder).