This is "Voice (over) IP" where IP is "Internet Protocol", the standards defining the operation of the underlying internet infrastructure, but often used to refer to any network based on these standards.
Originally Posted by CharlesMDietz
In most cases, people using VoIP are customers of a company operating a VoIP enabled telephone switch (or perhaps many of them). The end users cannot alter the ANI or caller ID numbers this way, short of a bug in the system they are using.
A business operating their own phone switch over a trunkline, whether that is done via VoIP or by PRI (Primary Rate Interface) can readily alter either the ANI value, or the caller ID fields (name and/or number) by "rigging the system". There is free source code available that can turn a Linux based PC into a phone switch, so this is, in effect, trivial to do even on a small scale of a single scammer in a small apartment who manages to pretend to be a big enough business to get the trunk connection.
Got an offer? Don't believe it unless it's in writing. You know the judge won't.
Be safe, avoid scams, never answer calls from callers you don't recognize.