I have actually dug into this a little bit, but mostly for our hosted Broadsoft platform... Tenacity has a partnered with Hamilton Captel to make a Cisco 8851 phone but it only works with Unified Communications Manager. I haven't found a "generic" product as of yet, but you can use a simple Grandstream ATA with existing CapTel style analog phones for the same experience. It is the best solution we have found, and in the Mitel it would be just a single line SIP device.