Modifying a button code for sending an sms

Hi All
The theme I'm using is called Tesseract

I am using Wordpress 4.4.2
It has a couple buttons in the header
I am trying to modify the code for one of the buttons so that when the customer brings up my website on a smartphone they can easily send me a text

WordPress or my theme keeps deleting my modifications
(When this happened before with "click to call" code it was because I wasn't entering the code properly) I think maybe this is happening again I have tried two different code configurations but none stick

The button code I am trying to modify is as follows

<a href="/" class="button secondary-button">Secondary Button</a>

Here are the two different modification I have made after href=

<a href="sms:5551212" class="button secondary-button">Secondary Button</a>

<a href="sms://5551212" class="button secondary-button">Secondary Button</a>

In both cases the modifications get deleted

What am I doing wrong ?

Thanks
Novice