Unfortunately, Gmail and Outlook.com do not support the SMS protocol used to create SMS links.
Gmail completely strips the HREF attribute when you use the SMS protocol. This happens not only in the mobile application but also in the web client.
Outlook.com modifies the markup deleting the entire link tag. Buttons may disappear from your email, remaining as only text strings without a link.
If an SMS link or button is your primary CTA, we recommend adding a fallback text string: "Button/link not working? Send an SMS straight through: +01 963658..."