[andyjohnson0]

I understand what this is but I'm having a hard time understanding how it works.

I understand that it is a sim that permits data-only connectivity for a large number of networks around the world. What I'm having difficulty with is understanding how the sim limits the user to using only whatsapp. Does the sim analyse network traffic content or endpoint to block non-whatsapp data? Is the whatsapp software supplied on the sim as an STK application that has sole access to the data channel? Something else?

What am I missing?

[pjc50]

Presumably the SIM does nothing special, it just directs the data connection to an APN run by the operator (like all SIMs do). This operator has firewall rules that drop everything non-whatsapp.

[kbart]

I guess they run as MVNO[1] and all data is routed through their APN. The rest goes as another comment has already described.

1. http://en.wikipedia.org/wiki/Mobile_virtual_network_operator

[ismail]

I seriously doubt this. They would need to sign MVNO agreements with every single operator they support. Unless they have a deal with someone that already has the links to the operators.