JSFiddle - React, Tailwind, and code Playground

HTML

<div class="phone" data-phone="01987123456">01987123456</div>

CSS

.phone {
    position: relative;
    text-indent: 1ch;
    white-space: nowrap;
}
.phone:before {
    background-color: white;
    border-right: 1ch solid white;
    content: attr(data-phone);
    display: block;
    left: 0;
    overflow: hidden;
    position: absolute;
    text-indent: 0;
    top: 0;
    width: 5ch;
}
@media screen and (min-width:0\0) and (min-resolution: +72dpi) {
    .phone:before {
        width: 5.8ch;
    }
}