I should be keeping this little secret to myself, but in the spirit of open sauciness and sharing:
Buy the Panasonic KX-HDV230 instead!
vs Cisco: everything just works without obscure settings and extremely long/complicated BLF configuration strings, which don't work when you switch to PJSIP anyway and cause days of troubleshooting.
vs Cisco: Cheaper than buying an SPA508G + 500ds attendant console
vs Cisco: You don't have to discover weird things like 'SPA50xG doesn't support EHS, but SPA525 does'.
vs Grandstream gxp2160: stuff works better, isn't slow, screen and menus aren't confusing.
vs Grandstream: gcp2160: headset remote answer doesn't delay ~6 seconds before answering the call
vs grandstream gxp2160: the BLF keys are LCD so your users don't have to tipp-ex out old names everytime somebody moves extension.
vs Yealink: See Grandstream.
vs all the above: these look and feel businessy and professional.
Only one issue left, and I will use my 'scope to figure out what's going on: There's no "ringing' in the earpiece of DECT Plantronics cs500 series (cs510 & cs520) headsets. EHS/remote answer works (without having to spend £25 - £50 on EHS adapter cables), but if you're sat on the toilet, you don't get the beep in the headset to tell you your phone is ringing. This is also a problem with the Grandstreams, which additionally, introduce a 4 - 6 second delay to remote-answer/EHS. Cisco 525g do the remote-ringing, and all Polycom SoundPoints seem to do it too (500, 501, 321, 330, 430).
They don't come with a PSU, and the Panasonic one is expensive. Where needed, a £5 + vat poe-injector-wall-wart works fine.