Not sure. But, you need to consider a few things
1) PhonePe is charging a certain platform fee depending on the transaction and frequency of transactions. Example- I was charged an amount of Rs 0.50 for a Credit card bill payment of Rs 28000, but was charged Rs 10 for a bill payment of Rs 10,000.
2) The success rate of PayTM for UPI is the highest among all apps. They also have faster UI response compared to any other app that I use.
3) I am very uncertain that PhonePe or any other app will share the exact API that they use- to avoid security breaches. But, most of them use the Bharat Bill Payment APIs for all their bills.