We have 3 VOIP numbers with Sipgate, this saves us about 90% of the cost of having those numbers with BT or VM and means we control the numbers and are not tied into any ISP bundle and are not tied to using the numbers at this building. We have Cisco VOIP kit.
Read a VOIP providers guides on configuring the VOIP phones to understand what is involved i.e. what you will need to do to configure a SIP account on a new VOIP phone to create a working service.
We started by registering one new VOIP service with Sipgate our choice of provider, once we had the first VOIP phone service configured & working we were in a position to request porting of a number from BT / VM etc. I would buy a VOIP phone or ATA of a type that the VOIP provider mentions, at least this way you know the device has a decent chance of working.
Only an active telephone number can be ported, that is a hard and fast rule.
