Cti Communications Ltd
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SIP VoIP to ISDN / ISDN to SIP gateway

Our ISDN to SIP converter gateway will let your ISDN phone system connect to VoIP - just plug it in. If your current ISDN phone system doesn't support SIP and you want to take advantage of cheap VoIP phone lines or any VoIP service without changing it then you need a SIP VoIP to ISDN gateway. Unplug from your BT ISDN box(es) and plug into the ISDN to SIP converter, your phone system doesn't know anything has changed.

We will configure the SIP VoIP to ISDN gateway with the details of your chosen SIP provider, you unplug from the ISDN & plug into the SIP to ISDN gateway and start saving money.

After lots of moving of goal posts these units have finally been added to a very small list of approved devices that BT has approved to connect to BT SIP VoIP services, and is by far easiest to implement as well as the cheapest ISDN system to SIP converter

Select the correct unit below to match your type of ISDN & the number of channel

ISIP 8 - 1 BRI (2 channels) 320 240
ISIP 8 - 2 BRI (4 channels) 480 300
ISIP 8 - 3 BRI (6 channels) 550 320
ISIP 8 - 4 BRI (8 channels) 660 420
ISIP 30 - 10 channels 820 660
ISIP 30 - 20 channels 1,010 840
ISIP 30 - 30 channels 1,200 960

The iSIP SIP VoIP to ISDN gateway connects to a PoE class 1 supply. If you don't have one we can supply them for 20 and you will need us to set it up and configure it for 20.


ISDN to SIP Converter

Customer Comment

After a year - "Great problem free product and we really appreciated your expertise"


Set the Firewall Rules

You will need to enable SIP and RTP port forwarding in your firewall, and lock them to your VoIP exchange. In a Draytek 2860 or newer this takes 20 minutes, if you want us to do this for you we can and will charge our normal support rates.

Ports Locked to VoIP Exchanges

For updates and provisioning Port 443 needs to access isdnsip.com. On other routers it may take longer to set things up.


When ready to test

  1. Unpack the iSIP converter and adaptor(s).
  2. Plug the iSIP converter into a PoE port on your network or plug it into a PoE injector and plug that into your network, watch the lights start flashing on the iSIP ISDN to SIP converter.
  3. Scan your network to find the iSIP converter, look for the MAC address or SpliceCom.
  4. Bind that MAC address to the IP address you want to use so the iSIP always receives the same IP address, if necessary, reboot the iSIP converter.
  5. Check the port forwarding rules in your firewall are set to this IP address.
  6. Look at that IP address in a browser and see if it is showing registered at the exchange.
  7. If registered, plug the adaptor(s) into the iSIP converter.
  8. Unplug your phone system's ISDN cable(s) from the BT box.
  9. Plug your phone system's ISDN cable(s) into the adaptor.
  10. Look at the converters IP address in a browser and it should show connected to your phone system.
  11. Try a test call, you should see via the browser page the number of calls seen on the ISDN and seen on the VoIP SIP connection.

How to connect iSIP converter to phone system


All prices exclude VAT, and were correct on 09/06/2022 15:28:59 , but should only be used as a guide.