SIP Basics

What is a SIP trunk?

A SIP trunk is a connection across the Internet that allows your team to make and receive regular phone calls. They can via the SIP trunk call to – and receive calls from – regular mobile phones and landlines worldwide.

SIP trunks are provided by service providers. They are used by Call centers and companies with IP-PBXs (business phone switches) as a way to connect to the traditional telephone network (PSTN).

The SIP trunk establishes a connection between the physical phone lines on the service provider’s side – and the business’s phone switch (software or hardware) on the other side. This is done using the SIP standard which is an open standard originally defined in the 90’s – which has become the de facto standard for Internet telephony across the years.

This gives you the ability to make and receive calls from the traditional telephone network, without the need to have a direct physical connection to the traditional telephone network yourself.



Connect the business phone system

The business phone system connected to the SIP trunk can either be a dedicated physical switch or software installed at your office location. You can also have the phone system software installed in some hosted solution – in the cloud – negating the need to have anything installed at the office.

Popular switches include Asterisk (an open-source switch), FreePBX (based on Asterisk) and 3CX.


Connect SIP phones

Call center agents and employees can use SIP-based phones to connect to the business phone switch. SIP phones can either be software apps or dedicated hardware phones. This gives them onward access to the SIP trunk and the traditional telephone network. This allows them to make and receive calls regardless of their location. They can use a headset together with a laptop or a mobile phone.



Connect local numbers to the SIP trunk

The SIP trunk provides you with the ability to receive calls via local phone numbers anywhere on the planet. Your team is able to handle calls independent of their location.

The capacity for concurrent incoming calls is restricted . Make sure that you have enough “channels” in your SIP trunk to handle the peak traffic you expect. This avoids the risk that callers get a busy signal during your peak hours. Some providers require that you have dedicated channels per phone numbers. Others provide shared channel capacity per country. Others – like Sonetel – provide a global shared capacity. This allows you to receive calls on any phone numbers worldwide as long as the total concurrent calls doesn’t exceed the capacity of the SIP trunk.

You can optionally pay per call minute for your incoming calls capacity.

Your team members can also make outbound calls with the local numbers. Some providers count the outbound calls as part of the “concurrent calls” allowed on the SIP trunk. Other providers (like Sonetel) do not apply the SIP trunk channel count on outbound calls.

If you have old phone numbers, they can normally be ported to the service provider, if number portability is supported in the country of the phone number.