Gmail and Outlook.com do not support the SMS protocol, which is used to create SMS links.
Gmail completely strips the HREF attribute when you use the SMS protocol. That happens not only in the mobile application but also in the web client.
Outlook.com modifies the markup deleting the entire link tag, so buttons may disappear from your email, remaining as only text strings without a link.
Those are known issues from both clients; because of this, when sending an SMS is the primary CTA, we recommend adding a fallback text string:
"Button/link not working? Send an SMS straight through: +01 963658..."
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