It depends how well you know the girl. I have paid by paytm even by Amazon gift card .. but then I know the girl last 5 years.
Most of the girls use 2 mobile phones. One which is for clients and second for her personal. Giving you the paytm number means giving you her personal number. Secondly they mostly don' tell their families they are working in this field. So to maintain secrecy they tend to deal only with cash.
But the girls are pretty smart themselves. If u look decent enough and promise to pay a big tip but short on cash at the spa, they will accept the money through paytm but they will give you the managers or counter paytm number. It happens in veda and a lot of other places. But only for known customers