A form field labeled "phone number" seems like it should be one of the simpler validation rules to implement — surely it's just a sequence of digits. In practice, phone number formatting varies enough across countries, and even within a single country, that naive validation logic built around one specific format assumption breaks for a meaningful share of genuinely valid real-world numbers.
Phone numbers don't have a single universal length
A common validation mistake assumes every phone number has a fixed total digit count — often based on whatever format is standard in the specific country the developer happens to be most familiar with. Actual phone number lengths vary meaningfully by country, and some countries even have variable-length numbers within their own system, depending on region or number type. A validation rule hardcoded to expect exactly ten digits, common for US-centric validation logic, will incorrectly reject a substantial share of genuinely valid international phone numbers that simply have a different, but entirely legitimate, digit count for their country's numbering system.
The country calling code isn't optional information, even though it's often omitted informally
In casual, informal contexts, people frequently write and share a phone number without its country calling code, relying on shared context (everyone already knows we're talking about a domestic number) to fill in the gap. But a phone number without an explicit country code is genuinely ambiguous in an international context — the same sequence of digits following a different country code can represent an entirely different, valid phone number in a different country. Software that stores or validates phone numbers without capturing the country code is implicitly assuming every user is calling from or dialing within the same single country, an assumption that breaks immediately for any genuinely international user base.
Formatting conventions — spaces, dashes, parentheses — vary by country and aren't just cosmetic
The visual formatting conventions used to display a phone number in a readable way — where spaces, dashes, or parentheses appear — vary considerably by country, and these aren't purely arbitrary cosmetic choices; they often reflect the actual internal structure of that country's numbering system, like where an area code boundary or exchange boundary falls. A validation or formatting rule built around one specific country's visual convention (parentheses around an area code, a specific dash placement) will produce output that looks distinctly wrong, or fail to correctly parse input, for a number from a country using different but equally standard and correct formatting conventions.
Why storing the raw digits and formatting only for display is more resilient
A more resilient approach that sidesteps most of these formatting-convention pitfalls is storing a phone number's underlying digits (including the country code) as the canonical stored value, completely independent of any visual formatting, and applying country-appropriate formatting only at the point of actually displaying the number to a user — rather than trying to validate or store the phone number in whatever specific formatted style it happened to be typed in. This mirrors a similar principle covered in an earlier post on this blog about storing dates and times as a stable underlying value (UTC) and only converting to a locale-appropriate display format at the point of actual display, rather than baking a specific formatting convention into the stored data itself.
Why a genuinely correct international validation library beats hand-rolled regex
Given how much real variation exists across countries in valid phone number length, structure, and formatting convention, a hand-written regex pattern attempting to validate "a phone number" in general, without being scoped to one specific known country's format, is almost always going to be either too permissive (accepting clearly invalid input) or too restrictive (rejecting genuinely valid numbers from countries the pattern's author didn't specifically account for). Dedicated phone number validation libraries exist specifically because they encode the genuinely complex, country-by-country rules about valid length, structure, and formatting that a general-purpose regex pattern realistically can't capture accurately across every country's numbering system at once.
The practical takeaway
If a form genuinely needs to accept phone numbers from users across multiple countries, capturing an explicit country selection alongside the number (rather than assuming one implicit country), storing the underlying digits independent of display formatting, and relying on established phone-number-validation logic rather than a custom hand-written pattern are all meaningfully more resilient choices than the common single-country-format assumption that trips up so much everyday form validation.