Asterisk - An open source telephony switching and private branch exchange service for Linux.
toolani - Cheap international calls
FreeSWITCH - FreeSWITCH is an open-source media application designed to support popular protools such as SIP and WebRTC and provides a platform to develop voice and video applications.
BNESIM - The all-in-one app for your communications. Voice, SMS, rooms, data SIM & eSIM in 200+ countries without roaming fees. Double data on the first top-up. Two years back-to-back winner of the "World Travel awards" for 2019 and 2020.
FusionPBX - FusionPBX is an open source FreeSWITCH GUI.
KeepCalling - Best Rates for International Calls. No Contract, Expiry, or PIN. Use Any Phone, Smartphone or PC.