Increasingly IP phone vendors are supporting the SIP RTCP Summary Report protocol. Vendors I'm aware of include Aastra, Nortel, Polycom and Snom - post a reply if you know of others.
The SIP RTCP Summary Report protocol provides a set of performance stats including packet loss, discards, burst/gap metrics, signal/ noise/ echo level and MOS scores/ R factors for the call. These reports are sent at the end of a call to a "Collector" (e.g. Telchemy's SQmediator). You can configure some IP phones to send interval reports during a call however this will increase the amount of data stored and should be used carefully.
If the IP phones at the two ends of the connection support RTCP XR then each can also include the "remote" metrics received in the RTCP XR message from the other end of the connection, giving a view of quality in both directions.