Case Study – Implementation of SIP

Our client, a large corporation, as a result of a cost saving and improvements exercise, required us to replace its PSTN PRI/BRI external voice communications links with a SIP voice system. The project was geographically controlled by the differing voice communications systems in place in the UK and in Europe and required us to make best use of data communications links and telco providers.

Essential Requirements:

The configuration of the voice networks resulted in a number of issues that moving to SIP based service would address, some of these were:

  • Management/overhead of supporting the gateway routers
  • Differing configuration requirements of the PRI/BRI configurations from differing Telco’s.
  • Differing call charges from the Telco’s leading to complex least cost routing configurations.

As such the following were designated as essential requirements:

For UK Offices.

In the UK all sites were using varying flavours of PSTN lines, ISDN PRI, ISDN BRI, from different Telco’s that were terminated locally on voice gateway routers

  • Replacement of all standard voice trunks.
  • Replacement of all local voice gateway routers.
  • All office external calls to be managed through SIP service.
  • SIP Gateways to be housed in Tier 4 Data Centre.
  • SIP Gateways to have resilience on hardware and have SIP trunks provided by two different Telcos.
For European Offices.

European offices whilst having a similar type of configuration also have a BT MPLS network and as such it made sense to use this network to provide access to the cloud based BT GSIP system for providing voice communications.

The following were essential requirements for the solution:

  • Replacement of all standard voice trunks
  • Replacement of all local voice gateway routers
  • All office external calls to be managed through BT GSIP Service


  • Establishing the configuration requirements from the differing local PABX/Voice systems.
  • Porting the DDI ranges from the various Telco’s to the SIP service provider.

What our Consultant did:

  • Established configuration requirements and documented change management process for each site.
  • Configured and implemented the SIP Gateways in the Data Centre ensuring resilience of hardware,connectivity and SIP trunk providers.
  • Established the DDI configurations with the Telco’s and produced change management process forporting of DDI’s to SIP providers.

Technology involved :

  • SIP
  • VOIP
  • Routing


  • Successfully implemented a highly resilient SIP service for UK sites and migrated all locally based PABX’s from using standard BRI/PRI PSTN trunks.
  • Successfully implemented BT’s GSIP service for all European Offices.


  • Cost
    • Reduction in call costs by standardising on SIP providers volume discounts.
    • Reduction in the trunk line rentals for both UK and European offices.
  • Management overhead – no requirement to manage the gateway routers at each site.
  • Availability
    • Highly resilient solution provided in the UK based solution by using Tier 4 DC with no singlepoint of failure.
    • Highly resilient solution for the European offices by using the cloud based BT GSIP solution.

Engineering Level Used:

Cisco CCIE Switch & RoutingCisco CCIE Security