Authoritative answer to all the questions: "it depends".
You couldn't even be bothered to specify what country you were talking about, reducing any attempt to respond to nothing more than a 'guess', at best.
In the U.S., 'local number portability' is a fact. With some caveats. Only the _owner_ of the number can 'port' it to a different carrier. Who actually 'owns' the number is not necessarily obvious. You _do_ probably 'own' the POTS or cellular number you use. If you have a personal 800 number, it is likely that the company providing the service 'owns' _that_ number, and has just 'loaned' it to you.
VOIP providers are the 'customer' to the telephone company, and, as such 'own' the numbers they were issued by the telephone company. They're just letting you 'use' one of their 'direct dial' extensions, as it were. (note: this situation has gotten a _lot_ muddier, with the gov't ruling that VOIP providers must provide '911' under the requirements for 'real' telephone companies. Argument can be made that if they're being treated as a 'real' phone company, they should have to do all the other things 'real' telephone companies do. Like number portability, 911 fees, 'universal access' fees, etc.)
There are a minimum of three players in any local number 'porting', possibly as many as _five_. (I think it's possible that one could even get more, but I cant think of how, right now.)
At a minimum, there is the 'customer' (you), the old carrier, and the new carrier.
'You' have to issue the authorization for the carrier change, the new carrier has to be willing (and able!) to handle calls for that number After those things are established, the old carrier *must* relinquish the number to the new carrier.
Things can get messy, since portability *is* only "local" -- you cannot 'port' a NYC number to Los Angeles, for example.
This means that the 'new' carrier must have physical equipment in the 'local' area of the original switch that serviced that number.
If the 'new' carrier doesn't have facilities in the right location, they're "not able" to receive the number.
The 'more complicated' case: you're a 'user' who buys service from the party (actual telephone 'customer') that 'owns' the number, and want to take it somewhere else -- where you'd just be a 'user' of a _different_ actual telephone 'customer'. (e.g., going from one VOIP provider to another VOIP provider.) This requires:
1) active co-operation from the current VOIP provider, to issue the LOA authorizing their _telephone_company_ to release the number to the designated carrier used by the new VOIP provider. 2) active co-operation from the new VOIP provider, to: (a) identify their carrier, (b) route calls to that number to _you_, and (c) co-operate in relinquishing that number if/when you decide to go 'somewhere else'. 3) a transfer of 'ownership' of that number from the current VOIP provider (remember _they_ are the telephone company's customer) to the new VOIP provider. This is necessary because the new VOIP provider doesn't use the same telephone company as the old VOIP provider -- thus the receiving telephone company does not 'recognize' the current provider (who issued the LOA for the change) as one of _their_ customers. And, similarly, the current telco doesn't recognize the new VOIP provider as one of their customers. 4) the telco of the new VOIP provider must have facilities in the right location, or they're "not able" to receive the number. 5) the new VOIP provider must have links to *that* equipment, or the telco must be willing to back-haul the connection from the 'destination' facility to the one where the VOIP provider has their connection.