If you care about voice quality and want a proper speaker phone, buy the SPA50X models. If you want the cheapest VOIP phone possible, get the SPA94X series.
Amazing speakerphone, lots of extensions, works with asterisk, configuration web page is upgraded from older models (SPA94x); Overall, it is a very good phone
Semi-pricey; Also, the 4 extra extensions are really overkill and you can't label them with the LCD, only the paper; I usually buy SPA508Gs instead of this one for that reason