Telecoms financed and built the communication infrastructure which made the internet (and thus, WhatsApp) possible. WhatsApp is also further subsidised because you also pay for the internet to use it.

I pay for an internet connection anyway. Even if I use a pay as you go mobile connection (on which the telco makes a profit) its still a lot cheaper to use Whatsapp than either SMS or voice calls.

There is no subsidy, its telecoms companies making a ridiculously high margin (aimed by regulators in some countries) on some services.