At least here in Brazil, WhatsApp consolidated its position by working well in feature phones that ran J2ME and dominated the low-end market before Android. It took testing and adaptation to each and every available phone (as far as I can remember, J2ME was kind of a loose standard so developing a J2ME on one device did not guarantee it ran well, or at all, in any other). This, coupled with the possibility of eliding the outrageous SMS rates and the availability of EDGE data plans.
WhatsApp ran on Nokia S40, which is J2ME, but it did not run on other J2ME phones (unless there was some way to put the s40 app bundle on them? I never heard about that).
There was a period of time where someone was selling low end phones with an unauthorized and incomplete client built in... In addition to not supporting group messages, it apparently had a backdoor to remote send spam.
