I have an SPA3102. I have never experienced DTMF issues, but I'm using firmware 5.2.13(February 2012). I'm aware of people using older firmware versions having issues with DTMF.
Audio quality on the PTSN line, when set to G711u, is perfect. G711u is G711u. 5.2.13 fixed a problem where the preferred codec setting was ignored. I can't speak to older firmware versions.