From their blurb from Mobcall for example:
Thinmail's newest service, Mobcall, provides instant push-button teleconferencing. It is triggered by sending an email message to any number of phones. They ring and whoever answers pushes a button and joins your conference call.
And similarly simple and interesting interfaces to all others services.
By conicidence, I also came to know of another service today - teleflip.com. Allows you to send SMS to any phone in US just by using email@example.com.