<label>
  <span>Телефон (без +7):</span>
  <input type="text">
</label>

body {
  min-height: 100vh;
  margin: 0;
  padding: 32px;
  box-sizing: border-box;
  display: grid;
  place-items: center;
  font-family: "Segoe UI", Arial, Helvetica, sans-serif;
  font-size: 16px;
}

label {
  gap: 0.75rem;
  display: flex;
  flex-direction: column;
  font-size: 1.5em;
  transition: all 0.2s ease-in-out;
}

label:focus-within {
  color: rgb(65 105 225);
}

input {
  padding: 1rem 2rem;
  border: 0.125rem solid rgb(51 51 51);
  border-radius: 0.75rem;
  font-family: inherit;
  font-size: 1.5em;
  color: rgb(85 85 85);
  outline: 0.25rem solid rgb(65 105 225 / 0%);
  transition: all 0.2s ease-in-out;
}

input:focus {
  border-color: rgb(65 105 225);
  color: rgb(0 0 0);
  outline: 0.25rem solid rgb(65 105 225 / 30%);
}

import IMask from "https://esm.sh/imask";

IMask(document.querySelector("input"), {
  mask: "000 000 00 00",
  lazy: false
});