// in the header.php file please find:
<?php if( mfn_opts_get('telephone-number') ): ?><p class="phone"><i class="icon-phone"></i><?php mfn_opts_show('telephone-number') ?></p><?php endif; ?>

// and replace it with:
<?php if( mfn_opts_get('telephone-number') ): ?><p class="phone"><i class="icon-phone"></i><a href="tel:<?php mfn_opts_show('telephone-number') ?>"><?php mfn_opts_show('telephone-number') ?></a></p><?php endif; ?>