callto

Callto is a URI scheme used by some voice over IP (VoIP) applications to initiate a call to a specified contact. A link that uses the scheme, such as callto:username or callto:[phone], is intended to launch the associated application and begin a call to the indicated contact if the application is installed on the system.

Origin and usage of the callto scheme are tied to early VoIP integrations, most notably with Skype. It is not part of a formal, standardized URI specification, and support varies by platform and application. Browsers or operating systems may register handlers for the scheme, allowing a click on a link to hand control to a installed VoIP client. Over time, the prominence of callto has declined in favor of other schemes and platform-specific connectors.

Syntax and behavior are generally straightforward but vary by implementation. The basic form is callto:<identifier>, where the identifier can be a username, a phone number, or a SIP address. Optional query parameters may be appended after a question mark to influence behavior, such as initiating a call or requesting video; however, the exact parameters are not universally standardized and are defined by the submitting application.

Compatibility and status: Because callto is not a universal standard, support is inconsistent across browsers, operating systems, and applications. Some modern communications ecosystems have shifted to alternative URI schemes (for example, skype:, tel:, or sip:), reducing the prevalence of callto links. When encountered, users should ensure the source is trusted, as launching external applications can have security and privacy implications. See also: Skype URI scheme.