system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do telephone numbers start with a 0?

I don't know if this is the case for every country around the world, but when I want to call someone, the first digit has to be a 0 if I'm calling someone within the country or 00[telephone code for country] if I'm calling someone abroad. 

Bonus question: why can the country codes (0049 for Germany for instance) be replaced by a "+" sign? (Like +49 instead of 0049)
gpt
In the old days, the first "0" indicated that that number was in a different locality - long distance calling. More accurately, the "0" indicates that "this subscription belongs to a different telephone exchange than this one". The second "0" means "Not only does this subscription belong to another exchange, it actually belongs to a different country!".

The reason for this is that old telephone systems were switched electromechanically with relays and such. When you dialed a number, a bunch of relays closed through a number of switchboards. If you started by dialing a "0", the switchboard opened up a connection to a long-distance phone exchange. If the first digit this higher-up exchange received was a second "0", the connection was routed to the international phone exchange. And these "traditions" are carried over to this day!

Pretty neat huh? We're still dealing with legacy from very old tech indeed, this way :)