Important - The Spectralink 8020 does not work properly with sipXecs.
I am leaving the documentation below in case Polycom resolves the issue or someone else wants to pick up this effort. Polycom resolved many of the issues that did exist between sipXecs and the 8020, but they introduced a new one. The phone now drops external calls at 1 minute and IVR calls at 30 seconds. I have quite working with those and moved to the 8400 line of phones. They run the Soundpoint firmware and work flawlessly. If someone thinks it is appropriate to modify this article or delete it altogether, please do. -Matthew Kitchin 9/7/11
Background
Spectralink 8020 wifi phones are full featured native SIP/WiFi devices that work well with Sipx. Sipx 4.2.1 and later require a version of 8020 firmware that as of 5/19/11 is not available without out calling Polycom support. It will then be provided free of charge. They want to track who all is using, but do plan to make it publicly available with their next firmware release.
Upgrading Fimrware
The 8020 needs to be upgraded to at least the firmware versions shown below. As of 5/19/11, you will need to contact Polycom support to obtain this firmware. Explain you are using Sipxecs and you need the firmware documented in Case# 1-103152086. This is the case that was opened specifically to resolve the incompatibilities between the Spectralink 8020 and recent versions of Sipxecs. With out this firmware, your 8020 will likely crash when answering an inbound call.
pd14csi.bin - 201.104
pd14odsi.bin - 119.080
pd14shim.bin - 119.080
pd14udsi.bin - 119.007
pi1400si.bin - 201.104
This guide assumes you have the charging cradle that allows you to update the firmware directly with the Handset Administration Tool (HAT). The HAT is available here: http://support.polycom.com/PolycomService/support/emea/support/voice/wi-fi/spectralink_8020_wireless.html
The current version is "SIP Handset Administration Tool (HAT) Software - WTB1xx and WTE 1xx (2.8.21.0)"
Power off your 8020 and put it in the cradle. The HAT program should then connect to the phone. Choose the firmware tab. Browse to each individual bin file, and select 'Update'.