I don't think it's particularly surprising or nefarious for WhatsApp to be paying carriers to become the default messaging client. That's just good business (unless you want to bring things like net neutrality into it).

The reason some exchange / sponsorship seems likely is that there's no option without WhatsApp. If you're a carrier, a GB WhatsApp data is the same load on your system as a GB of Telegram data. What's the incentive to make the most popular part of your product free?