BT solutions suggest - Business Communication Manager (BCM)
Developed by Nortel Networks, delivered and supported by BT, Business Communications Manager (BCM) is an advanced but easy-to-use, one box solution for simple telephony right up to a sophisticated mix of voice, data and Voice over IP.
BCM 400
Builds on BCM (200) Mini Office for organisations likely to grow beyond 32 users - expandable to 160 users)
Supports 8 Analogue lines, 8 ISDN2 channels and 30 ISDN30 Channels
Offers the same additional applications as BCM 200 - Professional Call Centre and DECT
BCM Power Office
A special BCM 400 package
Delivers voice and data capability for 50+ users, including unified messaging - expandable to 160+ users
Supports 30 ISDN30 (PRI) channels
64 extensions and the ability to move up to IP telephony without a big up-front investment
64 voice-mail user capability - so you're never likely to miss a call
64 unified messaging user capability, so staff can access and manage voice, fax and e-mail from a single interface even when they are not in the office
System-wide fax mailboxes and fax server and two Personal Call Managers
http://www.downloads.bt.com/b4b/pdf/BCM_brochure2.pdf
Or...
Cisco Call Manager Express (AVVID)
Range of features Internet Protocol (IP) enabled
Cisco AVVID (Architecture for Voice, Video and Integrated Data) is an IP-based communications system. Both data and IP telephony are supported on a common platform. It is ideal if you need IP telephony that provides basic voice requirements in a small site or branch office with up to 100 users. AVVID is modular and scalable from small to very large enterprises.
http://www.downloads.bt.com/b4b/pdf/avvid_brochure.pdf