Create a tel: link
Add block - Utilities - Social - Contact
This uses the tel: url scheme
Numbers should be in international format if you want to be able to receive calls from other countries
<xt:click2call tel="0123 456 789" text="Call me" />
Call me
The above example creates this link
<a href="tel:0123 456 789" class="xt_button">Call me</a>