6 is not optional: 
  correct: 0612345678, incorrect: 012345678
  - change: '6?' to '6{1}'

countryCode ('+31'), may also contain '00' instead of '+'
  correct, either: '+31612345678' || '0031612345678'
  - change: '(\+)' to '(\+|00)'

when specifying country code it's optional to provide '0' between braces before 6 (eg: '+31(0)6...')
  correct, either: '+31(0)612345678' || +31612345678
- change: ^(((\+|00)?31\(0\))|((\+|00)?31)|0)6{1}\d{8}$

same as previous commit