Kamailio.

Open-source SIP server for VoIP, WebRTC and real-time communications

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Asterisk
Open-source PBX framework for building voice and telephony applications
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CM.com
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Infobip
Croatian omnichannel communication platform with SMS, voice and chat APIs
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Bird (MessageBird)
Dutch CPaaS platform for SMS, voice, email and chat communication APIs
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NFON
German cloud PBX provider for business communications across Europe
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pascom
German business phone system with unified communications and team messaging
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Sinch
Swedish cloud communications platform for messaging, voice and video APIs
SE · commercial
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sipgate
German VoIP provider offering business phone systems and SIP trunking
DE · commercial
↗