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During the past year, sysmocom has been developing an open source fork of the popular Asterisk soft switch software which adds IMS Client functionality to it. IMS is the specification-language term for what is used to implement voice (and video) phone calls over 4G/LTE (VoLTE), 5G/NR (VoNR) as well as WiFi (VoWiFi).

Using this IMS-enabled version of asterisk and a SIM card in a smart card reader, you can make and receive voice calls using a cell phone number/subscription - without using any cell phone, cell phone network or even VoWiFi whatsoever. The IMS-enabled asterisk will establish an IP connection to the ePDG (evolved packet data gateway) of the cellular operator via any form of wired or wireless Internet access.

This can be useful for a variety of use cases, for example

• using a cellular subscription from a PBX - without needing any modem or cellular coverage
• research and development in the area of IMS and VoWiFi
• enabling VoWiFi from non-mainstream / alternative mobile phone operating systems

You can find a tutorial on how to set up and use this open source VoWiFi client at VoWiFi_with_Asterisk.

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