div.oevd-widget-calltoaction.roundbuttons .button .fo.fo-whatsapp:before {
    content: url("https://www.sparkassenversicherung.de/export/sites/svag/_resources/bilder/icons/whatsapp-icon.png") !important;
}

div.oevd-widget-calltoaction.roundbuttons ul li a.button .fo.fo-whatsapp {
    padding-top: 8px;
}