I did exactly that last month, got sick of paying the extra for a VM phone line, especially considering their ridiculous call charges. My VM vill went from £50 (50Mb + phone + unlimited calls) to £35 (50Mb).
Using a Siemens Gigaset N300A-IP which works with my existing Siemens DECT phones. Ported my VM telephone number to sipgate (£20 cost), use localphone for most calls, though sipgate are cheaper for 08 numbers, so use them for those.