Actually, I believe the reason for 555 was because that is a dead number. No matter what you dial, (555-1234 or 555-5678) you will not get connected. The Telco would set aside certain numbers for testing. There use to be a way where I could dial 550 plus the last four digits of my home phone and then hang up. The Telco equipment would send back a signal that would ring my phone. This was one way the Telco would check to see if the phone worked properly. There was another 551 or 552 plus the last 4 digits to check for rings, busy signals, etc. Now with all the new modern stuff, I amnot sure if all this stuff works anymore. By the way, I was told that if you go to Google,com and insert your telco number, it will pop up your home address. WITH a map if you ask for it. Pretty scary huh!
