HTML Guides for callto

When you write a phone link using <a href="callto:...">, you may encounter two distinct problems at once. First, the callto: scheme is a legacy, non-standard protocol originally associated with Skype. The correct and widely supported URI scheme for telephone links is tel:, as defined by RFC 3966. Second, spaces within URI scheme data are illegal characters. URIs must not contain unencoded spaces anywhere, and telephone URIs specifically expect a compact phone number composed of digits, hyphens (-), dots (.), and an optional leading plus sign (+) for international dialing.

The W3C validator raises this error because the value provided to href violates URI syntax rules. Browsers may still attempt to handle the link, but behavior will be inconsistent — some mobile browsers may not recognize callto: at all, and spaces in the URI can cause the number to be parsed incorrectly or truncated. Using the standard tel: scheme with a properly formatted number ensures the link works reliably across devices and platforms, including mobile phones, VoIP applications, and assistive technologies.

How to fix it

1. Replace callto: with tel: — The tel: scheme is the standard for phone number links and is supported by all modern browsers and mobile operating systems.
2. Remove spaces and slashes — Strip out any spaces, slashes, or parentheses from the phone number. These characters are not valid in a tel: URI without percent-encoding, and they serve no functional purpose in the link target.
3. Use a leading + for international numbers — If applicable, include the full international dialing code prefixed with + (e.g., +1 for the US, +49 for Germany). This makes the link work regardless of the caller’s location.
4. Optional visual separators — If you want visual separators within the href for readability in your source code, use hyphens (-) or dots (.), which are permitted in tel: URIs. However, the simplest and safest approach is digits only (plus the optional leading +).

Examples

Incorrect: callto: with spaces and slashes

This triggers the validator error because spaces and slashes are illegal in URI scheme data, and callto: is non-standard.

<a href="callto:07142/ 12 34 5">Call us</a>

Incorrect: tel: with spaces

Even with the correct tel: scheme, spaces in the phone number are still invalid URI characters.

<a href="tel:07142 12 34 5">Call us</a>

Correct: tel: with digits only

<a href="tel:0714212345">Call us</a>

Correct: International number with + prefix

<a href="tel:+490714212345">Call us</a>

Correct: Using hyphens for readability

Hyphens are valid characters in tel: URIs and can improve source code readability without affecting functionality.

<a href="tel:+49-07142-12345">Call us</a>

Displaying a formatted number to the user

You can still show a human-friendly formatted number as the visible link text while keeping the href value clean and valid.

<a href="tel:+490714212345">+49 (0) 7142 / 12 34 5</a>

This approach gives you the best of both worlds: the link text is easy for users to read, and the href value is a valid, standards-compliant tel: URI that works reliably across all devices and passes W3C validation.