Hello, go to: wp-content/plugins/geodirectory/geodirectory-functions and open the file custom_fields_functions.php
find this block around row 1950:
$html = '<div class="geodir_more_info '.$geodir_odd_even.' '.$type['css_class'].' '.$type['htmlvar_name'].'" style="clear:both;"><span class="geodir-i-contact" style="'.$field_icon.'">'.$field_icon_af.
$html .= (trim($type['site_title'])) ? __($type['site_title'],GEODIRECTORY_TEXTDOMAIN).': ' : ' ';
$html .= '</span>'.stripslashes($post->$type['htmlvar_name']).'</div>';
and replace with:
$html = '<div class="geodir_more_info '.$geodir_odd_even.' '.$type['css_class'].' '.$type['htmlvar_name'].'" style="clear:both;"><span class="geodir-i-contact" style="'.$field_icon.'">'.$field_icon_af.
$html .= (trim($type['site_title'])) ? __($type['site_title'],GEODIRECTORY_TEXTDOMAIN).': ' : ' ';
$html .= '</span><a href="tel:'.$post->$type['htmlvar_name'].'">'.stripslashes($post->$type['htmlvar_name']).'</a></div>';
Ps- I’m the author of the GD CLICK TO CALL widget 😛