• 02/xx xx xx xx (inside Czech Republic)
• +420 (0)2/xx xx xx xx (outside Czech Republic)

Actually not. The 0 has been dropped from the phone numbers some years ago. The original scheme was 4 to 8 numbers not starting with 0 for a local number (different districts had different number of phones and thus different number length) or 0 area_code local_number. 02 used to mean Prague. They increased the number of digits as necessary and made the area code required and thus the 0 unnecessary. So 02 12 34 56 78 became 212 345 678.

Regarding +420 (0)2 ... the original country code was 42 so the numbers were +42 0 area_code local_number. After we split with the slovaks our code was "changed" to 420, they were assigned 421. So the international numbers became +420 area_code local_number. In either case +420 02/xx xx xx xx would have one too many zeroes I believe. (Sorry for this lengthy and largely pointless post.)

In either case I think this is a bit too tricky task. Especially if you wanted them to accept the special numbers. 911 in US, 112,150,155,158 and probably a few others in Czech Rep and quite a few more in other countries. I think it's best not to be too strict while validating phone numbers. At least unless you can restrict them to a single country.