Why doesn't reactivity work in Vue (nuxt) attribute value?

Question

Vue.js

There is a code

<a v-bind:href="'tel:'+phone" >
{{phone}}
</a>

export default {
  data() {
      return {
          phone:  this.$GD('phone')
      }
  }
}

this.$GD('phone') - method from mixin.
the {{phone}} part is updated, but the value of the href attribute does not change.