I’ve had the same office phone extension for eighteen years; however, due to changing areas codes and exchanges I’ve had four different publicly-visible phone numbers during this time.
Actually, that’s not quite true, because at first internal dialing was four digits, and when we switched to the 936 exchange it became five. So my internal extension did change from XXXX to 6XXXX.
When I started at work they had run out of numbers in the 882 exchange and had started using 867. Having an 882 number marked you as an old timer. I only got my 882 number because I happened to occupy the office of a vendor who had been assigned the number, and instead of recycling it they passed it along to me. For some reason this bugged one of my then-housemates, who had started a couple of months before me but got an 867 number.
The final four digits in the 882 and 867 exchanges were in distinct ranges, hence it was possible to use four-digit dialing without conflicts. I used to know what those ranges were but it’s long since escaped my memory. I do remember that 5xxx was in the 867 range, which means that somebody at the company had the immortal 867-5309 as their office phone number.