Should I be wary of giving my cell phone number to matrixm.com?

I’m looking to get a real phone-sounding ringtone for my cell phone. I found a couple that I like on matrixm.com, but for some reason I’m nervous about giving them my cell phone number. Anyone that’s ever used them, have you had any problems with them?

And while I’m at it, do you have to “buy credits” to get any of their free tones?