A security precaution skipped in mobile applications such as Facebook’s Messenger could be abused to make an expensive phone call at a victim’s expense, a developer contends. Phone numbers often appear as links on a mobile device. That is possible by using a Uniform Resource Identifier (URI) scheme called ”tel” to trigger a call. URI schemes are a large family of descriptions that can tell a computer where to go for a certain resource, such as launching a mail application when an email address is clicked. Andrei Neculaesei, a full-stack developer with the wireless streaming company Airtame in Copenhagen, contends there’s a risk in how most native mobile applications handle phone numbers. If a person clicks on a phone number within Apple’s mobile Safari browser, a pop-up asks if a person wants to proceed with a call.