SIP Trunking Explained: Helping You To Deliver IP Telephony Services

SIP Trunking Explained: Helping You To Deliver IP Telephony Services

SIP trunking is playing an increasingly important role in the provision of communication and data services in modern business. With the BT 2025 Switch Off looming, in which the Public Switched Telephone Network (PSTN) will be permanently terminated, SIP trunking will be central to the effective switch from ISDN to VoIP technology, and a key asset for many IT businesses and owners.

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SIP (Session Initiation Protocol) trunking is something of a misnomer, as most people understand trunking to refer to a bundle of wires that are shared between phone users. Modern trunking, however, describes a virtual link between the PBX and the phone network, which uses an Internet connection – hence Cloud PBX SIP trunking will become more important and prevalent once BT terminates its ISDN and PSTN networks in 2025.


SIP trunks, which are the modern-day version of a T1 trunk, refer to a collection of channels that handle a certain number of calls simultaneously, rather than the physical cables that used to be required in telecoms systems. The number of channels that is required is determined by how many calls will be made and received at the same time; this can be calculated by the Internet Telephony Services Provider (ITSP) who supplies the SIP trunk and connects it to the VoIP/IP PBX.

SIP trunks offer several important benefits over traditional T1 trunks for businesses:

Cost efficiency: whereas businesses tended to have spare capacity with ISDN, for which they were charged, SIP trunks can be tailored to their precise needs to provide the necessary volume of calls. Line rental and calls tend to be cheaper, which particularly benefits businesses that make a high number of outgoing calls.

Scalability: SIP can be scaled up and down, according to usage, without the need to carry out extensive, time-consuming and costly network upgrading. Upgrading the capacity of SIP trunks is straightforward to provide more channels if required. New capacity is often available within 48 hours.

Unified communications: businesses rely on a variety of communications vehicles, including voice calls, video conferencing, email, instant messaging and voicemail. SIP trunking can effectively manage the entire portfolio, integrating more devices with the network (including mobile apps), and supports the migration of services to the Cloud.

Geographic flexibility: unlike traditional telephony which has posed restrictions based on location, SIP trunks allow for the selection of any number, irrespective of their geographical position. Offices can be relocated, even internationally, with no change to their numbers.


SIP trunks must be purchased from an ITSP before they can be rolled out to customers and connected to VoIP services, so joining a SIP trunk reseller programme is a potentially lucrative opportunity for IT companies seeking to expand their portfolio of telephony services.

With a transfer to VoIP guaranteed for over 2 million businesses in the UK as a result of BT’s 2025 Switch Off, now is the perfect time to become a SIP trunk reseller, so get in touch with us for more information.

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