A trunk line in a public switched telephone network connects the infrastructure of one carrier to that of another. Creating these connections enabled telephone networks to spread across the world, connecting any two people with a phone. The connected parties didn’t need to be registered with a single cumbersome central exchange in order to achieve a successful voice termination (a connected call).

PSTN also provided the means to record dialled numbers, call times and durations so that accounts could be settled between users and the different carriers.

SIP trunking or SIP (Session Initiated Protocol) is a protocol that enables the same services to be provided using an internet connection. Perhaps surprisingly, it has much in common with the Hypertext Transfer Protocol (HTTP) that transfers web-pages, and the Simple Mail Transfer Protocol (SMTP) that carries emails. It is also designed to work seamlessly with other telecommunications protocols such as UDP, TCP and SCTP.

By replicating all the physical lines and switches of a hard wired phone system in the Cloud, SIP is able to provide all the same services (including faxing, call forwarding, call queuing and text messaging) but with numerous advantages and additional abilities.

SIP trunking, voice termination and VoIP

Voice Over Internet Protocol is not as specific a term as you might think. Essentially, any connection that manages to carry voice across the internet can be called VoIP, however achieved. Most VoIP services simply resemble old style phones. They make limited use of SIP, which is capable of doing far more than merely replicating hardwired telephone networks.


SIP has been designed to enable multimedia and video streaming, video calls and virtual conferencing, file transfers, presence information, and even online gaming – all through the same channel.

One result is that communications become more reliable and versatile; text messages can be delivered by voice, voicemail can be delivered in an email, video chats can take place on mobiles, and you can easily join a team conference no matter where you are in the world. These different communication strategies can be integrated in a single application package. In turn, that can be integrated into CRM systems and enterprise application suites.

With improved communications, the need for face to face meetings may be reduced – saving senior staff travel time and hotel bills.


Whereas VoIP phone packages piggyback on office computers and their broadband routers, SIP phones just need an accessible modem. This makes SIP more appropriate for fully mobile devices.

The SIP trunking  platform is already creating a revolution in working patterns. Your staff can work together from distant offices, or from home. This is particularly emancipating for parents, allowing them to remain a valuable part of the workforce whilst caring for children.

Job candidates are increasingly interested in the options for remote working. VoIP solutions thus drive additional advantage in attracting / retaining valuable skilled staff.

Scalability and economy

SIP simply requires each user’s device to be equipped with SIP compliant software, whereas VoIP depends extensively on network connections, filtering and channel assignments being handled centrally. To an extent, VoIP still has some of the restrictions inherent in PSTN – you must have enough channels to avoid bottlenecks at peak times, even though most of the time you won’t be using them.

Although the virtual trunks of SIP superficially those of PSTN (for example each trunk has 24 lines) bottlenecks aren’t the same issue for SIP. You only pay for the trunk lines you use, so when demand increases you use more, but when it subsides you pay for less.

The cost savings over traditional phone lines are usually between 10% and 60% for most organisations. Heavier users save the most.


Your business may no longer need to invest in national 0800 numbers. Instead, you can set up local numbers at your busiest geographical locations, but still route all your calls to central locations with no capital investment because there is no additional hardware.


SIP trunking can be pre-configured to determine how to cope with emergency eventualities such as power or telecom outages. Calls will then re-route automatically to mobiles or alternative offices, and your customers need never know you had an issue.


With the increase in cyber crime and information legislation, some have concerns about the security of internet telecommunications. In fact, SIP offers better protection than PSTN. Tapping an ordinary phone line is, literally, child’s play. Simple devices at any point along phone cables can divert or record everything that passes through them. At one time you could bypass operators by whistling down the phone!

In contrast, SIP is routinely protected by network firewalls, IP filtering, encryption, number barring, login access, network security alerts, PBX security extras, line monitoring, detailed logging, and a variety of bespoke call restrictions.

Why not contact the experts at IDT today for further information on exploiting SIP.