The bluetooth on the iPhone is only used to connect to bluetooth headsets. There is no transferring files etc. with bluetooth. Other phones use a myriad of different filetypes for ringtones, some use mp3s, others use midi, etc. so it's not as simple as sending the ringtones to the phone. The iPhone uses m4r file type, so if the other phone uses mp3s you would have to send it to your email or somehow get it on your computer and then convert it to a ringtone with programs. I'm sure there are lots out there but I use a Mac so I just use GarageBand. If you want to find ringtones for the iPhone go here
Audiko - create free mobile and iPhone ringtones look for the search box and click the link underneath that says use audiko native search then input your search like "Transformers theme" , "SMS Tones", etc. it will take you to the results you can open each one you're interested in a different tab. If you like it there is a link that downloads the file directly as an iPhone ringtone you just save it wherever you want and double click the file it will then play in iTunes and be available for your iPhone. Connect your iPhone and go to the Ringtones tab and check the new ringtone and perform a sync and Voila it's on your phone.