I recently spent a couple of days troubleshooting a number of issues with our Cisco SPA525G wireless SIP phones.  After applying a number of updates to our UniFi access points, we started having problems with one of our wireless phones.  The web management interface was very slow or unresponsive and the call quality was noticeably worse.  Calls would get dropped and the phone kept losing its registration.  The problem turned out to be an issue with band steering.  Once band steering was turned off, call quality greatly improved and we no longer had issues with the phone registration.