There are two ways to connect Azure Communication Services to a third-party SIP:
- Voice Calling (PSTN)
- Azure direct routing
Voice Calling (PSTN)
This is a simple way to add PSTN connectivity to your app or service, with Microsoft serving as your telco provider. You can call phones all over the world by purchasing numbers directly from Microsoft. Because Voice Calling (PSTN) operates through Azure Communication Services, no on-premises deployment is required. If necessary, you can connect a supported Session Border Controller (SBC) via Azure direct routing to achieve interoperability with third-party PBXs, analog devices, and other telephony equipment supported by the SBC. Outbound calls with this option are charged at per-minute rates based on the destination country.
Azure direct routing
You can use this option to connect your own supported SBC to Azure Communication Services without the need for additional on-premises software. With Communication Services, you can use virtually any telephony carrier. You, your carrier, or your partner can configure and manage this option. It also allows you to configure interoperability between your telephony equipment and Communication Services, such as a third-party PBX and analog devices. This option necessitates an uninterrupted connection to Azure, the deployment and maintenance of a supported SBC, and a contract with a third-party carrier unless it is used to connect to third-party PBX, analog devices, or other telephony equipment for Communication Services users.