Your website uses HTML to render website to users' smart phone (browser)
HTML uses an ANCHOR TAG to render a clickable link
ANCHOR TAG uses attribute HREF to reference a resource on internet (a website page, phone/tel, email, etc.)
So, you will need to change the phone number to an ANCHOR TAG, using [tel] as the resource type in HREF attribute.
So, 123-45678 will become <a HREF="tel:123-45678">123-45678</a>
Happy to help.