SIP trunking vs traditional phone lines

The three types of line

Business phone systems have connected to the public network in three main ways. Each was designed for a different era, and many organisations still run a mixture of them.

Analogue lines (POTS)

A plain old telephone service line is a copper pair from the local exchange that carries one call at a time as an analogue signal. It is simple, powered from the exchange and compatible with almost any device, which is why analogue lines are still found serving alarm panels, lift emergency phones, door entry systems and fax machines. For a multi-line business, capacity grows one line at a time.

ISDN

ISDN delivers digital channels over the same network. Each 64 kbit/s bearer (B) channel carries one call, and a separate data (D) channel carries signalling such as caller ID and dialled digits.

• Basic Rate Interface (BRI): two B channels and one D channel, so two simultaneous calls per circuit. Common in small offices.
• Primary Rate Interface (PRI) on T1: used mainly in North America and Japan. A T1 has 24 channels; as an ISDN PRI, 23 carry calls and one carries signalling.
• Primary Rate Interface (PRI) on E1: used in Europe and much of the rest of the world. An E1 has 32 timeslots, of which 30 carry calls, one carries signalling and one is used for framing.

SIP trunks

A SIP trunk carries calls as IP packets over a data connection. There is no fixed relationship between the physical connection and the number of calls: capacity is a configured number of channels, limited in practice by the trunk agreement and by the bandwidth available. For a full explanation, see What is SIP trunking?

Side-by-side comparison

Capacity and scalability

With analogue and ISDN, capacity is installed. If an office on a 30-channel E1 needs 34 simultaneous calls, the answer is a second circuit, with the installation work and lead time that implies, and most of its capacity unused. Reducing capacity is equally slow, because circuits are usually rented on fixed terms.

With SIP, capacity is a setting. Channels can usually be added or removed without visiting the site, provided the data connection has room for the extra calls. That makes SIP well suited to seasonal peaks, campaigns, new teams and office moves. The constraint shifts from the number of circuits to bandwidth and network quality, so a move to SIP should include a check that the connection can carry peak call volumes alongside everything else.

Resilience and failure modes

Traditional lines and SIP trunks fail in different ways, and neither is automatically more reliable.

• Traditional lines depend on the physical circuit and the local exchange. A cable fault takes the lines out of service, and alternative routing depends on what the carrier offers.
• SIP trunks depend on the PBX, the local network, the internet or private connection, and the provider’s platform. In return, inbound calls can be sent to another site, a second PBX or mobile numbers when the primary PBX cannot be reached, and outbound resilience can be added with a second connection or trunk.

Resilience on SIP is therefore something you design rather than something you inherit. A sensible design covers power for the PBX and network equipment, a secondary connection where calls are critical, agreed failover destinations, and a test that proves each of them works.

Numbers and portability

Traditional numbers are bound to the exchange that serves the building. Moving office often means changing numbers, or paying to keep them diverted.

On a SIP trunk, numbers are routed to wherever the PBX runs: on site, in a data centre or in the cloud. Existing numbers can often be ported from the current carrier, which normally requires a letter of authorisation and an account holder name and address that match the losing carrier’s records. Portability depends on the country, the number type and the current provider, so it should be confirmed before any existing service is cancelled. Cancelling a line before the port completes can result in the number being lost. Number types and porting are covered in more detail in Understanding DIDs and virtual numbers.

How costs are structured

Prices vary too much by country, carrier and contract to compare in general terms, but the structure of the costs differs in predictable ways.

• Traditional lines carry a rental charge per line or per circuit regardless of use, installation charges, and call charges. ISDN primary rate services often have a minimum number of channels.
• SIP trunks are typically priced per channel or by usage, with separate charges for number rental and calls. There is no physical circuit to install, but the data connection must be adequate, and a session border controller or gateway may be needed.
• Migration costs may include PBX licences or upgrades, gateways for legacy equipment, network changes and porting fees.

A fair comparison sets the full cost of each option against your actual peak channels, numbers, destinations and call volumes, rather than comparing headline line rental with a headline channel price.

Copper and ISDN retirement

In many countries, incumbent carriers are withdrawing analogue and ISDN services as they move their networks to IP. Programmes typically stop new orders first and then switch off existing services exchange by exchange. Timetables differ by country and carrier and are sometimes revised, so check the published plans of your own carrier rather than relying on general dates.

For organisations still on ISDN, this turns migration from an option into a planning task. Starting early leaves time to test the PBX, deal with legacy devices and port numbers without pressure.

Migrating to SIP

A typical migration from ISDN or analogue lines follows these steps:

1. Inventory every line. Record each line and circuit, what it is connected to, and the numbers that ring on it. Include alarm, lift, door entry, fax and payment terminal lines, which are easily overlooked.
2. Check the PBX. Confirm whether it supports SIP trunks natively, needs a software upgrade or licence, or requires an ISDN-to-SIP gateway.
3. Size channels and bandwidth. Use busy-hour call data to set the channel count and confirm the data connection can carry it. See How to estimate SIP channel requirements.
4. Prepare the network. Configure firewall rules, NAT, the RTP port range and quality of service, and disable SIP ALG where it interferes.
5. Set up and test the trunk alongside the existing lines. Use temporary numbers to test inbound and outbound calls, caller ID, keypad tones and failover before anything is switched over.
6. Port numbers. Submit porting requests with matching account details and agree the cut-over date. Keep the old lines active until the port completes.
7. Cut over and review. Monitor early live traffic, resolve any issues, and only then cancel the remaining traditional services.

When to keep traditional lines

A complete move to SIP is not always the right first step. Keeping some traditional lines can make sense when:

• Life-safety devices, such as lift emergency phones or alarm diallers, rely on an analogue line and have not yet been certified on an alternative.
• The site’s data connection cannot carry voice reliably and cannot be upgraded in the near term.
• You need a simple fallback line that does not depend on local power or network equipment, where your carrier still provides one.
• A long fixed-term contract makes immediate replacement uneconomic, in which case SIP can be added for growth first.
• The service is still available from your carrier and your PBX is due for replacement soon, so it is better to plan both changes together.

Keeping lines should be a deliberate, time-limited choice where the service is being withdrawn. When you are ready to move, our SIP Trunking service covers compatibility checks, number porting and structured testing.