Multiple if else

How can i show some default code only if BOTH if statements are false ?

I've got this code -

  <?php if( get_sub_field('cta_phone')): ?>
   <a class="phone" href="tel:<?php the_sub_field('cta_phone');?>"><?php the_sub_field('cta_phone');?></a></span>
<?php else: ?><?php endif; ?>

<?php if( get_sub_field('cta_mobile')): ?>
   <a class="phone" href="tel:<?php the_sub_field('cta_mobile');?>"><?php the_sub_field('cta_mobile');?></a></span>
<?php else: ?><?php endif; ?>

<?php else: ?>
   <img src="http://my-domain/image.jpg">
<?php endif; ?>

i'm not using the else bit at the end at the moment because i only want that to show if both 'if's are fasle ?

hope that makes sense