Orbtalk have a range of SIP trunk products that work with the Cisco UC520
Cisco Unified Communications 500 Series for Small Business.
Use advanced communications capabilities including telephone call processing voicemail, automated attendant, and conferencing functions to respond to customers faster and save money on long-distance charges. Use with existing desktop applications such as calendar, e-mail, and customer relationship management programs.
Get an advanced phone system that expands as your business grows and changes, without a lot of capital expense.