ISDN PRI
An ISDN PRI delivers 23 bearer (B) channels plus 1 data (D) channel over a single dedicated 24-channel T1 circuit, giving a business 23 concurrent voice calls for PBX trunking. The PRI is stable, dedicated, and proven, which is why it served as the backbone of business telephone trunking for decades. It is now increasingly replaced by SIP trunking and VOIP.
How an ISDN PRI works
A PRI is provisioned on a dedicated T1 circuit and divides its 24 channels into 23 bearer (B) channels that each carry one voice call and a single data (D) channel for signaling. That gives 23 simultaneous voice calls between your PBX and the carrier. Because the PRI rides a dedicated T1, the trunk is stable with predictable, guaranteed call capacity.
Typical ISDN PRI pricing
When it fits
✓ Great fit
An ISDN PRI is a great fit for businesses running an on-site PBX that need a stable, dedicated trunk for up to 23 concurrent voice calls. It suits call centers, busy offices, and organizations that value the proven reliability of a T1-based PRI over a shared connection. Where a hard, predictable voice trunk matters, the PRI still delivers.
● Consider instead
Consider SIP trunking or VOIP when you want to scale beyond 23 channels easily, cut cost, or converge voice onto an existing broadband connection. The PRI is increasingly replaced by SIP because IP trunks add or drop channels in software and avoid a dedicated T1. If you are modernizing the phone system, VOIP usually offers more flexibility than a PRI.
How it compares
A PRI gives 23 fixed channels on a dedicated T1; SIP trunking scales channels in software over broadband and costs less, which is why it is replacing PRI.
A PRI is a dedicated TDM voice trunk; VOIP carries calls as IP packets over broadband for lower cost and richer features.
A voice T1 provisioned as a PRI is the same 24-channel circuit; the PRI specifically signals 23 B channels plus 1 D channel.
What to look for
- ✓23 B plus 1 D. A PRI dedicates 23 bearer channels to calls and one D channel to signaling, for 23 concurrent voice paths.
- ✓Dedicated T1. The PRI rides a dedicated 24-channel T1, so trunk capacity is guaranteed, not shared.
- ✓PBX compatibility. Confirm your PBX has a PRI card and supports the carrier's signaling before ordering the trunk.
- ✓Migration path. Since SIP trunking and VOIP are replacing PRI, ask how the trunk maps to IP if you modernize later.
Common questions
What is an ISDN PRI?
An ISDN PRI is a digital voice trunk that delivers 23 bearer (B) channels plus 1 data (D) channel over a dedicated 24-channel T1, supporting 23 concurrent voice calls for PBX trunking.
How many calls can a PRI handle at once?
A single PRI handles 23 concurrent voice calls, one per bearer channel, while the 24th channel carries signaling as the D channel.
How much does an ISDN PRI cost?
An ISDN PRI typically costs between $350 and $900 per month depending on call volume and whether bundled long distance is included.
What is the difference between a B channel and a D channel?
On a PRI the 23 bearer (B) channels each carry one voice call, while the single data (D) channel handles call setup and signaling.
Is a PRI dedicated or shared?
A PRI is dedicated. It rides its own 24-channel T1 circuit, so its 23 voice channels are reserved for your business and not shared.
Is ISDN PRI being phased out?
Yes. The PRI is increasingly replaced by SIP trunking and VOIP, which scale channels in software over broadband at lower cost.
When should I choose a PRI over SIP trunking?
Choose a PRI when you want a proven, dedicated T1 voice trunk with fixed, guaranteed capacity; choose SIP trunking when you want lower cost and channels that scale on demand.
More on ISDN PRI
An ISDN PRI, or Primary Rate Interface, is a digital voice trunk built on a dedicated 24-channel T1 circuit. The PRI splits that T1 into 23 bearer (B) channels plus 1 data (D) channel: each B channel carries a single voice call, while the D channel handles signaling and call setup. The result is 23 concurrent voice calls between a business PBX and the carrier, delivered over a stable, dedicated circuit. Because the PRI rides its own T1 rather than a shared connection, trunk capacity is predictable and guaranteed, which made the PRI the workhorse of business phone trunking for decades. A PRI typically costs in the range of a few hundred dollars per month for the full 23 channels.
The PRI's strengths are reliability and dedicated, guaranteed capacity, but it is increasingly replaced by SIP trunking and VOIP. IP trunks add or remove channels in software and ride existing broadband, so they avoid the cost and rigidity of a dedicated T1 PRI. Businesses keeping an on-site PBX still buy PRI trunks where proven stability matters, while those modernizing migrate the same 23-channel capacity onto SIP. Either path connects the PBX to the public phone network.
Types of serviceOptions and variants
What businesses use it for
- ✓PBX voice trunking. A PRI connects an on-site PBX to the carrier with 23 concurrent voice channels on a dedicated T1.
- ✓Call center trunks. The PRI's 23 bearer channels handle high simultaneous call volume for inbound and outbound centers.
- ✓Direct inward dialing. A PRI carries DID number ranges so calls reach extensions directly through the PBX.
- ✓Stable business telephony. Where guaranteed, dedicated voice capacity matters, the PRI delivers a proven 23-channel trunk.
- ✓Bridge to SIP migration. Many businesses run a PRI today and map its 23 channels onto SIP trunking as they modernize.
ISDN PRI details
An ISDN PRI provides 23 B channels and 1 D channel. The line is a dedicated 24 channel T1 circuit providing an internet connection straight from the broadband provider backbone. This makes an ISDN PRI a very stable and reliable connection. Depending on your location and T1 provider availability, your ISDN PRI connection will normally be priced between $350 and $900 monthly
The connectivity ladderHow it compares, and where to go next
Where this connection sits among the options, with the businesses each one fits.
| Service | Connection details |
|---|---|
| Business DSL | 256 kbps to 1.5 mbps connection - Business DSL is a shared line for 5 to 50 internet users. |
| Fractional T1 Connection | |
| Integrated T1 Connection | |
| Full DS1 / T1 Connection | |
| Multiple T1 Lines | |
| DS3 / T3 Connection | 45 mbps connection - Dedicated line for 100+ users and/or high bandwidth applications. A DS3 / T3 connection can also be used for point to point access. |
| OC3 Connection | 155 mbps connection - Fiber or dark fiber line for enterprise applications. An OC3 connection can also be used for point to point access. |
| OC12 Connection | 620 mbps connection - Fiber or dark fiber line for enterprise applications. An OC12 connection can also be used for point to point access. |
| VPN Solution | Managed hardware VPN solutions for secure private network connections and VPN server software for secure virtual private networks. |