I use heytell on my iphone...I'm assuming it's similar??
Same thing basically.
Cell phone companies don't really care about whether you're using talk or data minutes, they're selling it all to you at markup you wouldn't pay for oil from mars and it's all going over the same network anyway. Voice is digital too.
For example at average SMS pricing you're paying a penny per
byte -- if you use all 160 characters per SMS, more per byte if you don't -- and they're getting that penny both ways (from both parties). The markup is astronomical across all cell services, SMS just makes an easy comparison point.