JavaScript to add country codes to all phone numbers without them, for the Mac Script Editor.

Add Country Codes.scpt

{
  const countryCode = '+44'

  const addCountryCodeToPhoneNumbers = person => {
    for (const phone of person.phones()) {
      if (phone.value().startsWith('0')) {
        const newValue = phone.value().replace(/^0/, countryCode)
        console.log(`${person.name()}: ${phone.value()} → ${newValue}`)
        phone.value.set(newValue)
      }
    }
  }

  const contacts = Application("Contacts")
  for (const person of contacts.people()) {
    addCountryCodeToPhoneNumbers(person)
  }
  contacts.save()
}