What type of programming do you do? Because in my experience, programming is as much an art as it is a science (or mathematics). In my previous job, I had to classify phone numbers as either 1) valid NANP [1], 2) invalid NANP [2], or international, and it wasn't clear cut like balancing equations. I was surprised that the Oligarchic Cell Phone Company (our customer) didn't even send us valid phone numbers!

[1] North American Numbering Plan, 10 digits, with a three digit area code, a three digit exchange and a four digit number. From what I was able to understand (there wasn't a single document that described this), the three digit exchange had to start with 2-9, and the second two numbers couldn't be 1 (so no area code like 211, 311, 411, etc., but 201, 210 were valid), and the same for the three digit exchange, EXCEPT maybe for toll-free numbers (like 800-xxx-xxxx or 866-xxx-xxxx) where the exchange could be 211, 311, 411 (I never did get a clarification on that point).

[2] A number that might present itself as a NANP but doesn't follow the actual NANP specification.

[3] In telephony, there are two concepts, international numbers that can be dialed world wide, and "local", which can only be dialed from inside a region (like North America) or country. A number that starts with a "+" is considered international, which should be followed by a country code (one to four digits, starting with 1 to 9, with no rhyme nor reason as to length). If it doesn't start with a "+", it's a "local" number, limited to, as I stated, a region or country.

There is also no requirement that a "local" number be related in any way, shape, or form, to an international number. For instance, a "local" NANP number like 305-867-5309 can be represented as a international number as +1-305-867-5309, but for several countries, just taking the "local" number and adding a "+<country code>" won't work. Different rules for different regions/countries.