Downloading music from your PC's music collection is pretty easy. All you have to do is drag and drop songs from your PC onto a memory card that's then installed in the phone.All the phones connect to PC USB cable. However only Sprint / Nextel and Amp'd phones appear as a lettered drive onto which you can drag and drop music files.
Not everything on your PC will necessarily play on the phones. Sprint phones will play MP3's and AAC's ( the default format for Apple's iTunes software and Music Store) but not music purchased form other music stores even PC downloads from the Sprint service, oddly enough. With Amp'd you can play MP3's and WMA's but not songs downloaded from online music stores. AMP'd Live does allow you to order songs, ringtones and other content from your PC and have them automatically pushed to you phone.
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Verizon is the most "Apple unfriendly". It's phones don't play MP3's or AAC's the only formats used by iPod owners just WMA's. The only good news is that Verizon phones will play copy-protected WMA songs bought from other online stores (which of course rules out Apple iTunes content). I hope this helps you in terms of what cell phone you have now or are contemplating on purchasing. Below is a place that I found to be helpful in this process. Check it out.