Before making any commitments, customers often will search a series of acronyms and ask questions “is SIP and VoIP the same?”, “How does VoIP work with SIP?”, “what is VoIP?”. It is common for businesses to be inundated with confusing telecoms abbreviations from ISDN, PSTN, PBX, and so on. 

In today’s telecoms environment, quite often customers mix SIPs and VoIP definitions, this can be easily done as they achieve the same goal in connecting calls via the internet and work coherently. 

IP – Internet Protocol

VoIP – Voice over IP

SIP – Session Initiation Protocol

Trunking – Grouping

ISDN – Integrated Services Digital Network

PSTN – Public Switched Telephone Network


SIP is the acronym for Session Initiation Protocol. The communication protocol businesses often use in every day running for initiating, maintain, and terminating real-time sessions that include voice, video, and messaging applications.

SIP Trunks connect your PBX to the BT network, enabling full PSTN breakout on the public telephone network.

Connection from your customers site (or sites) to the network via an IP connection (for example broadband or Ethernet) and is delivered as an end-to-end service with an availability guarantee, voice channel guarantees and voice Quality of Service.

SIP Trunks provide a highly flexible alternative to ISDN and are compatible with all the leading IP PBX brands in the UK market, giving you peace of mind that your PBX hardware works with the network service.


VoIP, Voice over Internet Protocol, allows users to make phone calls over LANs (local area networks) or the internet. The technology being VoIP converts voice into digital packets, that are sent across a network using Internet Protocol (IP) to the endpoint. Connecting VoIP phones and your VoIP service provider to the VoIP PBX. Even though a VoIP phone system uses VoIP and is connected to your LAN most systems can connect directly to the PSTN. Giving businesses the ability to use both PSTN and VoIP for calling. Upon all VoIP systems, VoIP phones systems are easily integrated into the telephony network. 

In simplest of terms, IP/VoIP calls are calls made over the internet, SIP is the protocol used to make those calls and connect multimedia sessions, with VoIP calls just one type of session they can handle.

The differentiator between SIP and VoIP is that VoIP is limited to transferring voice data over the internet, whereas SIP can transfer packets of multimedia data using voice, video, or text.

Moreover, VoIP calls happen only over the internet or a private internal network, however, SIP trunk can transfer data packets over any network, for example, VPN, ISDN, or internet. Making it much more versatile for businesses and customers. 

PSTN Switch Off 2025

SIP and VoIP is now replacing the traditional ISDN and PSTN services, with circuits no longer orderable, in anticipation and response to the Nationwide PSTN 2025 switch off.

While PSTN and ISDN used to offer the fastest internet access available (128kbps) it is now a relic of a bygone era, with faster internet access connections like DSL and WAN far surpassing the speeds that PSTN and ISDN lines are available to deliver cost-effectively. Essentially, it’s an outdated legacy system now. 

PSTN and ISDN networks have enhanced and upgraded over the years but the fundamental network itself has largely remained unchanged. This, in conjunction with the rise of VoIP (Voice over Internet Protocol) as a more cost-effective and future-proof alternative, resulting in the death of ISDN. 

Two big contenders who provide the answer to the PSTN & ISDN switch off are VoIP and SIP solutions.