SIP trunking is a service enabling companies to link their on-site IP PBX (phone system) directly to an Internet Telephony Service Provider (ITSP), granting access to a range of services including inbound phone numbers and outbound call termination. Essentially, it serves as a modern replacement for traditional PSTN phone lines. SIP trunking can function via both public internet and private IP network.

Rather than focusing on contrasting features, it's crucial to understand how these services complement each other. SIP trunking facilitates the connection of your VoIP-compatible on-site phone system to the cloud, enabling communication with the external world. Hosted PBX, on the other hand, is already a phone system, operating entirely via the Internet. By integrating them, you can leverage some.

SIP trunking supersedes outdated PSTN and ISDN phone lines by leveraging the internet for voice data transmission. Key benefits include:

1. Lower cost compared to traditional PSTN and ISDN connections.

2. Scalability, allowing for easy adjustment of the number of channels or simultaneous calls required.

3. Easy relocation, as you can use your SIP Trunk from any location with internet access.

4. Enhanced security features, SIP trunks can be configured to encrypt all call data during transit.

5. Support for HD voice codecs for superior audio quality.

A SIP channel facilitates either an incoming or outgoing call, akin to a traditional phone line. Each SIP trunk can accommodate multiple SIP channels.

The number of channels a SIP trunk can handle depends on your internet connection speed. Most modern connections can support dozens of channels. For instance, a 10 Mbps connection could sustain approximately 100 simultaneous calls.

This depends on your on-site PBX and its supported codecs. If your PBX supports G.729, you only require 0.03 Mbps including overheads. Therefore, if you have a reliable connection of even 8 Mbps, you could have about 260 simultaneous calls connected.

Even if your internet connection goes down, you'll still be able to receive inbound calls. This is possible by setting up a call diversion to external phone numbers. Typically, calls first route to an inbound number and then pass through the SIP trunk. However, if the SIP trunk is unreachable due to internet unavailability, calls will automatically redirect to a configured mobile phone or alternative number.