Thank you for your quick answer; if more info req'd, then FCC-ID=52PT-3511 (so model is PURETEK PT3511,quite old) and chipset is Rockwell R6785-61. The model is well-known, drivers are not too difficult to find; the modem works correctly as transmit/receive device, even with the "generic HCF driver" included in XP.
But no VOICE-SPKRPHONE function is available...just the only one useful for me !!!