<a href="https://wa.me/549351XXXXXXX" target="_blank" 
   style="
     position: fixed;
     bottom: 20px;
     right: 20px;
     background-color: #25D366;
     color: white;
     border-radius: 50%;
     width: 60px;
     height: 60px;
     display: flex;
     align-items: center;
     justify-content: center;
     font-size: 30px;
     text-decoration: none;
     z-index: 9999;">
   ☎️
</a>