Orbtalk have a range of SIP trunk products that work with Cisco Call Manager
Cisco Unified Communications Manager (formerly Cisco Unified CallManager) is the powerful call-processing component of the Cisco Unified Communications solution. It provides voice, video, mobility and presence services for businesses with up to 30,000 users. Unified Communications Manager is a scalable, distributable, and highly available enterprise-class IP telephony call-processing system.
Cisco Unified Communications Manager creates a unified workspace that extends enterprise telephony features and capabilities to packet telephony network devices such as IP phones, media processing devices, voice over IP (VoIP) gateways, mobile devices, and multimedia applications.