Samsung and T-Mobile decided to lock ringtones with some crappy DRM, so you have to buy their ringtones, and not just use some random clip of a song you like. Good luck if you don't like prime time TV clips and Top-40 music.
They missed something. They let you use .3gp files as ringtones, which as guides on the web would have you believe, are 8-bit, 8000Hz and abysmal quality.
I discovered however that the 3gp file format supports the AAC codec, so you can make a decent quality tone with
ffmpeg -i infile.mp3 -ac 1 -ab 36 -acodec aac outfile.3gp
Take that, silly carrier.