Upvote:2
All Dutch telephone numbers have 10 digits, starting with the 0, (which falls away when you add the international number.)
So for a Dutch mobile number you will have +31 6 ABC DE FGH. Whereby the letters stand for the digits of the telephone number.
The 6 is for a Dutch mobile number, a Rotterdam land line number starts with 010, (so +31 10 AB CD EFG).
Made up numbers can be excluded by tests, sites have ways to check that you use a real number, like sending a text message asking you to reply or do something on the app.
Checking that the phone is in Rotterdam and nowhere else is much harder.
All Dutch mobile numbers and many foreign numbers can be in Rotterdam.
If it is possible to get a location from the network, which they can do if the phone is set to allow it, you could use that. Or you could ask the phone owner to enter extra details which show (as far as you can confirm) he is in the city.
PS, the spaces I put in the numbers are for reading only, while dialing you keep the numbers following, no spaces.