SIP trunking has been transforming business communications for years, yet misconceptions persist. Whether you're considering replacing ISDN or expanding your telephony, let's separate fact from fiction.
SIP (Session Initiation Protocol) trunking delivers voice calls over your internet connection instead of traditional ISDN lines. It's now the standard for business telephony — but old myths die hard. Here are the 8 most common misconceptions we encounter, and the reality behind each one.
This is one of the most persistent myths and it couldn't be further from the truth. Businesses that switch from ISDN to SIP trunking typically see a reduction of 40–60% in their telephone bill. But the savings go far beyond call costs.
SIP trunking eliminates the need for expensive ISDN line rentals, reduces hardware costs, and offers per-second billing with no minimum call charges. You gain unlimited concurrent channels — no more paying for capacity you don't use. You can also consolidate multi-site telephony into a single platform, reducing management overhead and simplifying your infrastructure.
Beyond cost, SIP trunking delivers operational gains: faster provisioning (add channels in minutes, not weeks), geographic flexibility (keep numbers when you move offices), and built-in disaster recovery through automatic failover.
This myth dates back to the early days of VoIP when internet connections were slow and inconsistent. In 2024 and beyond, with modern broadband, dedicated Ethernet connections, and Quality of Service (QoS) guarantees, SIP call quality is equal to or better than ISDN.
Blue Telecoms supports HD voice codecs including G.722, G.722.1, and CELT, which deliver crystal-clear audio that surpasses the standard ISDN call quality most people are used to. With proper network configuration and a reputable provider, jitter, packet loss, and latency are negligible.
If you've experienced poor VoIP call quality in the past, it was almost certainly a network issue or a low-quality provider — not a limitation of SIP technology itself.
With a competent provider, setting up SIP trunking is straightforward. In most cases, it involves configuring your PBX or phone system with a handful of settings — SIP server address, credentials, and codec preferences — and you're live.
Blue Telecoms has tested and verified compatibility with all major PBX platforms including Asterisk, Avaya, 3CX, FreeSWITCH, and SNOM. We provide configuration guides, and our technical team handles the setup with you if needed. Most clients are up and running within hours, not days.
Compared to ordering, provisioning, and installing ISDN lines — which can take weeks and require BT Openreach visits — SIP trunking is dramatically simpler and faster to deploy.
SIP trunking works anywhere you have an internet connection. Unlike ISDN, which requires physical copper lines to a specific location, SIP is location-independent. Your office could be in central London, rural Wales, or your spare bedroom — if you have broadband, you can use SIP.
Modern SIP solutions also support NAT traversal, which solves the historical issues with firewalls and routers that used to cause connectivity problems. Blue Telecoms provides full NAT traversal support, mobile access, and even supports voice encryption for government and public sector deployments.
With remote and hybrid working now the norm, SIP's location independence is actually one of its greatest strengths — your team can take calls from anywhere with the same quality and the same numbers.
In the early days of SIP, there were genuine interoperability concerns between different vendors' implementations. Those days are long gone. SIP is now a mature, well-standardised protocol, and any reputable provider will have tested their service against the most popular PBX systems and hardware.
Blue Telecoms supports a wide range of codecs — PCMA/U, G.722, G.722.1, CELT, GSM, Speex, iLBC, and T.38 for fax — ensuring compatibility with virtually any SIP-compatible device. We've tested with Asterisk, Avaya, 3CX, FreeSWITCH, SNOM, and many more.
If you have an existing PBX, the chances are it will work with SIP trunking out of the box. If there's an unusual configuration required, our technical team will help you get it working.
This is the opposite of reality. SIP trunking is almost always significantly cheaper than ISDN. With ISDN, you pay for every line regardless of whether you use it, plus per-minute call charges with minimum call durations. With SIP, you get unlimited concurrent channels and per-second billing.
There's no hardware to install, no line rental in the traditional sense, and no BT Openreach fees. Blue Telecoms offers free SIP trunking with unlimited concurrent channels and built-in PSTN failover. You only pay for the calls you make, billed per second with no minimum charge.
For most businesses, switching to SIP trunking reduces their telephone costs by 40–60%. For high-volume environments like contact centres, the savings can be even greater.
Modern SIP trunking is at least as reliable as ISDN — and in many cases more so. Unlike ISDN, which relies on a single physical connection that becomes a single point of failure, SIP trunking can be configured with automatic failover to backup routes, mobile numbers, or alternative offices.
Blue Telecoms includes built-in PSTN failover with our SIP trunking service. If your internet connection goes down, calls are automatically rerouted so your business never misses a call. Our UK-based servers deliver high availability, and we monitor our infrastructure 24/7.
With ISDN, a line fault means waiting for an engineer visit — often days. With SIP, failover is instant and automatic. That's not less reliable — it's more.
Some people believe that SIP trunking only works if both sides of the call are using SIP. This is completely false. SIP trunking connects to the public telephone network (PSTN) just like ISDN does. Your callers don't need SIP, VoIP, or any special equipment — they can be on a landline, mobile, or any phone system in the world.
When you make or receive a call via SIP trunk, the connection between your system and your provider travels over the internet using SIP. But at the provider's end, it's connected to the PSTN — the same telephone network that every phone on the planet connects to. The person you're calling has no idea (and doesn't need to know) that you're using SIP.
You can present any DDI number — geographic (01/02), UK-wide (03), freephone (0800), or even international numbers — and the call will reach any phone number globally, just as it would from an ISDN line.
Blue Telecoms provides free SIP trunking with unlimited channels and built-in failover. Talk to our team about migrating from ISDN or setting up a new SIP service.