Great question — answering as the UphillTech team:
Twilio can call a number of endpoints including phone numbers (PSTN), SIP, and Twilio Client (Javascript, iOS, and Android). Your question includes an Android tag. If Android is the platform of interest, try using the Twilio Voice Android SDK : Twilio Programmable Voice SDK for Android allows you to add voice-over-IP (VoIP) calling into your native Android applications.
Full answer by jassent: https://stackoverflow.stackexchange.com/a/77526275 (CC BY-SA 4.0).
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