The KPN Mobile boo-boo was its attempt to surcharge its subscribers for using WhatsApp. That lead to howls of outrage and then directly (no passing Go, certainly no collecting £200) to Europe's first net neutrality law. (see - KPN in hot water for double-DPIng smartphone users).

Well WhatsApp is back and causing more sleepless nights for telcos (see - SMS in decline but telcos should be able to protect the bottom line). Since April when it hit 200 million users and pronounced itself bigger than Twitter, it's grown by a half to 300 million. That's doubly surprising in light of the fact that it recently announced a plan to surcharge itself. Instead of 'monetising' its free service with advertising it's monetising it with money - charging users 99c after a year of use and 99c every year for every after that. The fact that it has decided to charge and not to plaster annoying targetted ads to the interface, only seems to have endeared it further to its loyal users.

Obviously 99c is a tiny amount, even for poorer people in the emerging economies, but it doesn't take an accountant to work out that it may represent $300 million and still rising in revenue. Even if WhatsApp finds it loses, let's say, a third of its users when it comes time for them to pony up the 99c, that's still exceptionally good business considering the theoretically minimal costs of supporting the service.

If a fully-formed IMS mobile phone user can be supported in the cloud for 2 cents per user per year (see - Network innovation and transformation: by how much will SDN/NFV crush costs?) then even if WhatsApp cost twice as much (and why would it?) we're talking about a gross profit margin of 95 per cent.

And of course, just to make sure it keeps all those single dollars and builds many more, the company is adding more services. WhatsApp has just announced a simple push to talk option - the perfect accompaniment to the standard messaging offer.