How ISDN works
ISDN transmits digital signals over standard copper wiring. There are two main variants:
- BRI (Basic Rate Interface) - basic connection with two voice channels (2B+D), suitable for smaller companies
- PRI (Primary Rate Interface) - thirty voice channels (30B+D), designed for larger companies with a high volume of calls
Each B channel carries one call. A company with a BRI connection can therefore handle at most two calls simultaneously, to increase capacity, an additional physical line had to be added.
Why companies are leaving ISDN
Operators in many countries are gradually phasing out ISDN networks. Companies are switching to SIP trunking for several reasons:
- Flexible capacity - The number of simultaneous calls for SIP trunks can be adjusted without a technician's physical intervention.
- Lower operating costs - VoIP calls are generally cheaper than fixed ISDN lines.
- Geographical freedom - telephone numbers are not tied to a physical connection point
- Modern supplementary functions - You can easily add non-geographic numbers, DID extensions or 800 numbers to your SIP trunk.
Transition from ISDN to SIP trunk
Switching from ISDN to a SIP trunk does not require replacing existing telephones or the physical PBX. It is sufficient to adjust the PBX configuration, instead of the ISDN interface, set up the SIP trunk.
What the transition entails:
- Existing telephone numbers can be ported to a SIP trunk provider.
- The internet line must have sufficient capacity and voice traffic priority (QoS) configured.
- It is advisable to check whether the router supports the problematic SIP ALG and, if so, disable it.
On virtualni-ustredna.cz, you can navigate the transition from ISDN even without deep technical knowledge.
Frequently Asked Questions
When will ISDN be definitively discontinued?
The date depends on the specific operator and country. We recommend checking your operator's current plans and not underestimating the preparation for the transition to SIP trunking: the transition usually takes several weeks.
Can I transfer an ISDN number to a SIP trunk?
Yes, phone numbers can be ported. The transition does not require changing the numbers customers already know: the numbers will remain the same.
Will call quality change after switching from ISDN to a SIP trunk?
With correct QoS settings and a sufficiently sized internet connection, the quality of VoIP calls is comparable to or better than ISDN. A stable connection with low latency is key.