Is there a "faxto" or similar protocol?

Question

I have a client with a fax number, and after linking their address to Google Maps, their email with a mailto:, and their phone with a callto:*, I felt wrong leaving their fax number unlinked. Is there any way I can link a fax number?

* I would use tel:, as that is an actual standard, but callto: is more supported

Solution

UPDATE: As indicated by @Jukka, the fax URL scheme from the rfc2806 specs has been obsoleted by RFC 3966. So you should just be able to use tel: for Fax numbers also.

From the RFC 3966 Specs - http://www.ietf.org/rfc/rfc3966.txt

The identifier approach described in this document has the disadvantage that certain services, such as electronic banking or voicemail, cannot be specified in a "tel" URI.

The notation for phone numbers in this document is similar to that in RFC 3191 [RFC3191] and RFC 3192 [RFC3192]. However, the syntax differs as this document describes URIs whereas RFC 3191 and RFC 3192 specify electronic mail addresses. RFC 3191 and RFC 3192 use "/" to indicate parameters (qualifiers). Since URIs use the forward slash to describe path hierarchy, the URI scheme described here uses the semicolon, in keeping with Session Initiation Protocol (SIP) URI conventions [RFC3261].

The "tel" URI can be used as a request URI in SIP [RFC3261] requests. The SIP specification also inherits the 'subscriber' part of the syntax as part of the 'user element' in the SIP URI. Other protocols may also use this URI scheme.

The "tel" URI does not specify the call type, such as voice, fax, or data call, and does not provide the connection parameters for a data call. The type and parameters are assumed to be negotiated either in-band by the telephone device or through a signaling protocol such as SIP.

This document obsoletes RFC 2806.

OBSOLETE ANSWER

Yes. Use it like so.

<a href="fax:[phone]>+415.123.4567</a>

Depending on Global or Local number, you use the + or * prefix. Recommend you use the global notation.

The tel, fax and modem URL Schemes are discussed in rfc2806 specs https://www.ietf.org/rfc/rfc2806

Here's the relevant section

2.3 "fax" URL scheme

The URL syntax is formally described as follows (the definition reuses nonterminals from the above definition). For the basis of this syntax, see [RFC2303] and [RFC2304].

  fax-url          = fax-scheme ":" fax-subscriber
  fax-scheme       = "fax"
  fax-subscriber   = fax-global-phone / fax-local-phone
  fax-global-phone = "+" base-phone-number [isdn-subaddress]
                     [t33-subaddress] [post-dial]
                     *(area-specifier / service-provider /
                     future-extension)
  fax-local-phone  = 1*(phonedigit / dtmf-digit /
                     pause-character) [isdn-subaddress]
                     [t33-subaddress] [post-dial]
                     area-specifier
                     *(area-specifier / service-provider /
                     future-extension)
  t33-subaddress   = ";tsub=" 1*phonedigit