NO !
Your local exchange may have the basics of the countries prefixes, and cut you off from dialling toward a non-existent exchange or range. So you dial 85 and it knows , thats not allocated, you cant dial more. But its only going to be for exchanges, like the first 3 digits out of 10 … its not going to do the check of many digits for numbers at other exchanges… it might cut you off for nonallocated numbers at your own exchange.
Now as for dialling more than the number of digits required… your POTS (analog) circuit that is run by an ISDN based exchange is going to know how many digits you should dial for a normal number, get that number from you, AND THEN connect it through as an ISDN call … so if you have to dial 8 digits, it makes the call when you dial 8 digits.
But if you use a digital system,where you put the number in and THEN send the call, you can put more digits in, and the ISDN will happly send through all the digits you put. Then its up to the ISDN based PABX to make use of the extra digits, or just ignore them…
Now how do exchanges send numbers through to the correct VOIP system ?
ISDN supports hair pinning… it refers to hairpin U turns. Say I’m your local exchange, and I used to have your number. But you are now on voip. Then I’m configured to hair pin the call… its directed to the my telephone companies central equipment… a big smart exchange that is will look up a database and pull out where you are to be found, and so forward the call … its done by adding prefixes, so hence the need for ISDN to just send through all the digits of a number… redirection means it grows larger…its redirected to a magic phone number… but since its got your original number appended… ISDN sends through heaps of info, but this is the result , or a possible result, its up to the telephone company to agree with the other company on how to redirect calls to VOIP , for portability … etc… but in concept , its done as a serious of redirections…
I think ISDN understands that if a call is HAIR PINNED, the data is no longer to be sent to the device issuing the HAIR PIN instruction… so the voice data takes the short cut after that. so a telephone companies central call router can handle 20 million calls an hour, but even if all the calls are an hour long, it doesn’t have to handle 20 million calls at a time. Its hair pinning them to other companies or to other exchanges in the same company…
So really your local exchange doesn’t have to be told you have moved your number to VOIP. Its already just sending all calls to unknown numbers to the central exchange, which will have a database lookup for number porting, to VOIP providers,other teleco’s mobile phones,etc.