The MNO Econet Wireless in Zimbabwe has what are termed social media data bundles, these data bundles only work on say Twitter, WhatsApp, Instagram, etc. When you purchase say the WhatsApp bundle, you get assigned some MBs of data that only works on WhatsApp. You can text, view statuses (even video statuses), download/upload media, etc. However, you cannot make call (either voice or video calls).

If you initiate a call, the call goes through, it will ring for the other person but as soon as they answer, your WhatsApp will be stuck on the "connecting" screen. The same is true for the reverse i.e. if someone else calls you, you get stuck with a "connecting" screen as soon as you answer.

Aside from it being very annoying that my "240MB" of data is apparently unable to make a call on WhatsApp, I am generally curious how they might be doing this, the simplest explanation I have is they limit the bandwidth so much that a call cannot go through.

Whatever method they are using to do it, I imagine it can also be used to "fingerprint" someone. It's not a lot of information, but it can be a useful/helpful data point if coupled with other data.