>what is the cheapest/simplest gsm-cellular which
>connects to a PC via a serial [not USB] cable, which is still
>'available' ? With a modem [9600 Bd = OK] and able to
>dial into my existing land-line ISP also ?
That would be CSD, and would only work if your carrier supports CSD.
See how CSD works in the FAQ below.

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Best regards, SEE THE FAQ FOR CINGULAR WIRELESS AT
John Navas <http://en.wikibooks.org/wiki/Cingular_Wireless_FAQ>