It would be useful to add VoIP numbers to the database where available. This should be an up-and-coming area, though pretty restricted at the moment. It's possible for most VoIP users to call most others anywhere, totally free of charge. At the moment it's complicated. There are 2 basic ways:
1. If the called party has a fixed IP address from the Internet provider, that number can be called over VoIP. Exactly how depends upon the caller's hardware
. To call 123.186.204.123 a Grandstream VoIP box requires dialling *47123*186*204*123#; a Draytek VoIP router #123*186*204*123#. (For the expert, a non-static number can be called with the help of DDNS - the whole subject is long and complex).
2. If the caller and the called number use SIP providers (companies like Internet service provides), it is possible to call between many combinations of providers. The details, of course, are specific to each pair of providers (the computer industry loves standards, that's why there are so many of them).
and 3. And there are proprietary systems like Skype, which talk to other Skypeists but not SIP.
Having said all this, if a VoIP contact is published it's unique. How to call it is messy, not like picking up a phone and dialling a number: you have to work out what to do. (If it were easier everyone would do it, free; bad news for phone companies!).
As a start, for specialist Internet, email and VoIP company Gradwell:
SIP: sales@gradwell.com
SIP: support@gradwell.com
SIP: dsl-support@gradwell.com
SIP: porting@gradwell.com
SIP: billing@gradwell.com
(I don't use them, but can call them free if I need to).