The ITU provides the answers you need, in various forms. But captured well here -
https://www.itu.int/ITU-D/treg/VoIP.pdf where it says...
"In India, VoIP is allowed, but only for computer-to-computer communications. India deregulated IP Telephony on 1 April 2002 following the ITUís World Telecommunication Policy Forum held in 2001 on the topic of ì IP Telephonyî. Indiaís proposed unified licence regime, however, would impose no restriction on VoIP telephony or other IP-enabled services, provided they are offered by operators with a unified licence that have duly paid all required registration charges."
..In practice, this means Twilio can terminate a call into India, as long as the route the call takes is via a licensed operator. This is Twilio's problem, not yours. (unless it says otherwise in their terms).
It also means if an individual, physically within India, chooses to make an outbound call, over the Twilio service, then the endpoint has to be another IP-endpoint. Or if to a 'copper cable endpoint', then at least traverse over an appropriate Indian carrier at some point. Probably using some kind of PSTN-IP gateway. Again, Twilio's problem and not yours (unless it says otherwise in their terms).
These complications were introduced to prevent people in India setting up copper cable-IP gateways, taking advantage of cheaper retail call bundles, and selling them cheap on the wholesale market. Naughty!
Personally, I would contact Twilio. They won't be able to give legal advice, but they will at least tell you if they can legally facilitate the requirement.
