insane: Indeed, I believe a whole host of retailers are still making use of dialup based EFTPOS terminals.
While the DSL based EFTPOS is far faster, it is by no means more reliable so dialup still has its place. Its also rather handy for those cisco out of band dial in devices used to administer remote equipment.
Dialup EFTPOS works fine over VoIP providing you're using uncompressed codecs (ie ulaw or alaw).
The issue isn't one that modems can't work at all over VoIP as ulaw/alaw have been the core of the digital phone world for decaudes. The problem is one of reliability over VoIP networks where many people are looking at low bitrate codecs such as G729 and where issues such as jitter and packet loss have to be factored in.