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Put the following in the EMBED CODE option of the INSERT edit function. Change the number and the text to what you want it to be.
<a href="tel:+1-303-499-7111">Click to Call NIST Time Service!</a>
<a href="sms://+14035550185">Send an SMS</a>
Click and drag the buttons to resize the window.
You can also use SKYPE by entering:
<a href="skype:echo123?call">Call the Skype Echo / Sound Test Service</a>
<a href="skype:SKYPE USER NAME?call">Call me on Skype</a>
The visitor must have the Skype app installed for the Skype link to work.
For the button I used this code:
<a href="tel:+1-303-499-7111"><img src="https://dabuttonfactory.com/button.png?t=Call+Me&f=Calibri-Bold&ts=24&tc=fff&tshs=1&tshc=000&hp=20&vp=8&c=5&bgt=gradient&bgc=3d85c6&ebgc=073763" title="303-499-7111" /></a>
Unfortunately this doesn't work with the Google Sites built-in buttons.
To use a Google Drive image get its URL and modify it as shown here:
<a href="tel:+1-303-499-7111"><img src="https://drive.google.com/uc?export=view&id=1zCDQ5ze8Cih491-8FRlLLlaYmMQdxJlN" width="60" title="303-499-7111" /></a>
<a href="sms:+1-4035550185"><img src="https://dabuttonfactory.com/button.png?t=Send+Text+Message&f=Calibri-Bold&ts=24&tc=fff&tshs=1&tshc=000&hp=20&vp=8&c=5&bgt=gradient&bgc=3d85c6&ebgc=073763 "title="403-555-0185" /></a>
The number in this example is a fake number that doesn't go anywhere.
You can also use Google Hangouts. Insert the URL 'hangouts.google.com'. You will be able to select to make a phone call or send a text or make a video call.
You can create a QR Code image for dialing. There are several QR Code generators.
Search Google for "qr code generator"
Create a QR Code with the phone number format.
Upload it to your site.
Viewers with a QR scanner on their phones can scan it to bring up their dialer.