We bought the phone from Voipon.co.uk and to be fair we haven't as yet asked them the direct question. That is today's question.
Research suggests that there are major differences between an S450 and a C450/460. The S450 (now discontinued) is a SME product whereas the C450/460 is aimed more at consumers. The C460 is the phone we actually have (although the web server reports the base station as C450).
The C450/460 is a dual line product -1 x PSTN and 1 x SIP