@JoshRoss'phones' can cover a multitude of things.

If you are talking analogue phones connected to the PSTN via POTS, then the grunt work of encode/decode is being done at the exchange, so even the simplest analogue handset will work ok.

However, if you are talking about office-type SIP-VOIP (as opposed to say, Skype-VOIP) then the handset (or soft-phone app) is doing the encode/decode work, and so there can be a vast difference in call quality depending on the device being used, not to mention all the other variables like Firewalls, Routers, PBXs etc.

I've installed SIP phone systems using CISCO and POLYCOM handsets, and the call quality is first-class, better than a classic analogue call, while the same PBX using a Lynksys SPA phone makes the caller sound like Darth Vader with a head-cold.