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 The Business Phone Blog


Using SIP Trunks as an ISDN replacement

We have had quite a few customers recently comparing the cost and quality of ISDN vs using a combination of SIP trunks and SHDSL,  so we thought we would run a comparison for cost using a ISDN 30 as the high end benchmark and 2 x ISDN 2 as the low end benchmark.


An ISDN 30 PRI Line rental would be typically around the $900 mark and comes with the carrier installation, fixed at 30 channels (it can be varied to 10 or 20 channels), and carrier call pricing which would be around 20 - 35% higher than typical SIP Trunking call charges.

Using a 2mb/2mb SHDSL connection (this should carry 30+ calls) the cost would be, depending on carrier selected,  between $350 and $599 and the cost of 30 SIP trunks would be $300 so over all, the rental is similar or within $150 per month if using a budget carrier. Note budget services will not provide the full advertised bandwidth so call quality may suffer during peak times.

So why consider using SIP Trunking as an ISDN replacement? The main difference is flexibility. The SIP trunks can be added or removed as required depending on workload and call volume. SIP trunks can be added and removed with a phone call with next to no lead time. A 2Mb/2Mb service can carry up to 40 channels. SHDSL comes with substantially lower installation costs than ISDN and that would remain as the only contracted portion of the service.

So overall it may be a borderline decision Replacing ISDN 30 with SIP trunks until greater penetration of SHDSL leads to price reductions for SHDSL. However if your business needs on demand flexibility in the number of channels required for call volumes then SHDSL should be the choice.

When it comes to call charges there is no comparison ... SIP Trunking with un timed national calls, no flag fall on calls to mobiles and flat rate locals calls makes SIP Trunks around 20 - 35% lower.


Substituting SIP trunks for ISDN 2 may be a simpler and more cost effective solution. With an ISDN line rental being around $75 per month we will use for the exercise 2 X ISDN 2 services.

To replace 4 channels of ISDN only requires an ADSL service and 4 channels can easily be run on a ADSL connection shared between Voice & Data...meaning because most business already have an ADSL connection there is no added cost for running 4 SIP trunks over the ADSL connection.

With the cost for 4 Sip trunks being around $50 vs $150 for 2 X ISDN 2 it seems to be a no brainer to use SIP trunking as a substitute for ISDN 2 and gains the benefit of significantly greater flexibility with the capacity to take SIP trunks up or down in 2 channel increments at any time.

The only limitation is with existing PBX equipment the customer will need to get ATAs (typically $100 per channel) and possible card changes to the phone system to make SIP trunks work. Of course if starting  out new equipment can be ordered ready to go.  


Its a simple but lengthy exercise to port your current phone numbers to SIP Trunks to take advantage of lower call costs and line rentals. SIP Trunking provides customers with access to Business Grade VOIP services at lower cost than PSTN or ISDN and provides for dramatically cheaper phone calls.

For the customer there is next to nothing in it. All the work is done by your telco. You just need to provide your Telstra account number, phone number and business name & ABN and a port to SIP Trunking can be undertaken.

The process works as follows.

1. Provide your details for the line you want to port to a SIP trunk as described above.

2. The Telco submits the SIP Trunk port request to the carrier.

3. In the Interim speak to your PBX maintainer to ensure your PBX is set up and compatible for SIP Trunking.

4. It typically takes 5-10 weeks for a PSTN port to take place. You will be given a weeks notice of the port date and time that the SIP trunk will be ready to use. There is typically about 20-60 minutes down time. Ports can be planned for after hours at additional cost.

5. Prior to the port your PBX maintainer can set up the SIP trunk using a domain, number/username and password on your phone system.

6. When the port take's place your SIP trunk is ready to use.


One of the big advantages with SIP trunks when compared to the average PSTN or ISDN phone line is ease to set up.

With SIP Trunking its especially simple to add and remove channels. We received a call this morning from a customer needing 4 SIP trunks for their PBX Phone System. Within 90 minutes of getting the call the customer was supplied with their Username and numbers, password and domain and had the SIP trunks up and in operation.

When they move they only need an ADSL connection to continue. When they want to down size they call us and we disconnect the service within minutes. When they want to up size they call us and we add extra channels within hrs ...not days or weeks as is typically the case with a PSTN or ISDN line.

Whats more their cheap. Its free to connect a SIP trunk and only $12 a month per channel compared to $30+ for ISDN and PSTN Lines and there's no need for a lengthy contract.

Flexibility is the word when it comes to SIP trunks.


We regularly get asked whats the difference between the higher cost quality SHDSL Internet carriers and the budget SHDSL services that are about. Are they the same? The answer to this is a resounding no!

So what do you get with with the cheap SHDSL plans? Well, you will get an ‘Up to 10Mb’ service with no guarantees around bandwidth, that can only be used for browsing (that’s in the T&Cs for that product with most no guarantee on capacity for Hosted Voice or SIP Trunking) and is provided by a supplier that is nowhere near as organized operationally as the the quality SHDSL carriers that Telaustralia uses. The up to 10Mb is the key here – with Telstra BDSL Standard, if you sign up to a 10Mb, you get a 10Mb (allowing for minimal loss in IP overhead). With the budget carriers in Australia you’ll get whatever can be provisioned across 4 wires and can be allowed within distance. Where Telaustralia's SHDSL carrier will install extra ULLs if required to get you to a 10Mb speed. With the budget SHDSL carriers if you If you want to get extra ULLs you have to pay more… To the point that the "budget" pricing is back in the ballpark of the quality carriers ...without the quality!

What is also true is you pay more for the quality SHDSL plans that Telaustralia provides and as always, it  will be provided and sold as a premium business grade product with a whole set of guarantees around bandwidth, support, response and restoration.

So in the case of SHDSL services, you really do get what you pay for.


Our new ADSL 2 bundled plans are available now. What do you need to do to take advantage of our great bundles...sign on with your Landline and Internet for 24moths to get this fantastic value.

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