IP Phones are not part of the BT Hosted IPT Product, but for clarity are discussed here. IP Phones should be obtained from BT in order for correct network configuration to take place.

Telephone Handsets - To the end user is your organisation, the telephone is the most visible component of the VoIP network. BT currently only recommends the use of Cisco IP Phones for use with the BT MM VoIP service. They have been designed specifically to support the underlying platform architecture and run proprietary protocols to facilitate ease of use. Currently, there are several models of IP Phone to choose from - from basic reception phones, through to advanced display models and 802.11b compatible wireless phones - all of which are available on the Cisco product portfolio.
Each Cisco IP Telephone provides toll-quality audio and doesn't require a companion PC.  Because it is an IP-based telephone, it can be installed anywhere on the Customer’s corporate LAN or WAN IP that has QoS connectivity to the MM VoIP service.
With VoIP port, you can keep your existing handsets. For further information on Cisco IP Phones, please click here >>

IP PBXs - The only currently supported IP-PBX is the Cisco AVVID. For IP Voice, we offer a bundled solution for the Customer Premise Equipment based on the BT Managed CPE model.  The Distributor will manage the maintenance, monitoring and reconfiguration of the CPE through BT and the global BT CPE supplier.
The recommended router is fitted with voice cards to compress the voice and convert it to IP Data connected to a dedicated Ethernet port on the BT WAN Customer Premises Router.