tel: prefix in the href attribute modifies the link behavior that varies with device capabilities:
- Cellular devices autodial the number.
- Most operating systems have programs that can make calls, like Skype or FaceTime.
- Websites can make phone calls with
registerProtocolHandler, such as web.skype.com.
- Other behaviors include saving the number to contacts or sending the number to another device.
Since we never know where visitors to your site are coming from, we need to call phone numbers from anywhere in the world. Therefore, the + sign is always required. The + sign is automatically converted to the international dialing prefix by your mobile service provider.