What is the Difference between an SPA series phone and the 6900, 7900 and 800 series phones? by Acoustical Innovations | Monday, June 3, 2013 | 0 comment(s) The main difference between these two different types of phones is the connection type. . The SPA series allows for a a special 2.5mm adapter while the Professional series requires the use of an RJ-9 adapter. This entry was posted in Headsets for Cisco Phones. 0 Comments Add a Comment to this Post You must be logged in to post comments.