This conversation should really be in the Non-Mitel Chatter section, I know this seems odd since those are Mitel phones, but they are just devices, most of the discussion areas here are for Mitel Phone SYSTEMS (5000, 3300, SX200, etc. are all Mitel branded phone system servers), the fact that some of the phones have 5XXX model numbers does not relate to the forums.
Mitel phones in these areas use proprietary firmware that is NOT SIP compliant, but use the MiNet protocol... and all the firmware that we use in general is specific to the software revision of the phone system they are connected to, and phones on the SX200, 3300, and 5000 all use different firmware packages and are not SIP compliant.
At one time Mitel released and supported "dual mode" phones (like your 5220 set), but they discontinued the phones many years ago, and supported and software was discontinued a few years later. The software used to be freely available from Mitel, but it is hasn't been for about 4 years or so. The later software revisions are very hard to find, but for tips look at this thread:
http://mitelforums.com/forum/index.php/topic,2800.0.html or
http://www.voip-info.org/wiki/view/Mitel+SIP+Firmware for more information.
I don't mean to be just put push you off, but most of us in these forums never did use the SIP firmware and actually saw it as a way to dumb-down a Mitel phone to make it compatible with 3rd party systems. The effort never took for Mitel either, and they closed the book on that chapter of their history.
Looking or asking in the FreePBX, Asterisk, Kamailio, or other SIP based PBX forum will likely get you more information and the files you need quicker and with more experience than here... Sorry.