phone.html

<link rel="import" href="bower_components/polymer/polymer-element.html">
<script src="https://unpkg.com/string-mask@0.3.0/src/string-mask.js"></script>
<dom-module id="ns-phone">
    <template>
            {{label}} <input type="text" value="{{value::input}}">
    </template>
    <script>
        class NsPhone extends Polymer.Element {
            connectedCallback() {
                super.connectedCallback();
            }
            static get is() { return 'ns-phone'}
            static get properties() {
                return {
                    country: {
                        type: String
                    },
                    strings: {
                      type: Object,
                      value: {
                          en: {
                              label: 'Phone'
                          },
                          de: {
                              label: 'Telefon'
                          },
                          es: {
                              label: 'Telefono'
                          }
                      }
                    },
                    rules: {
                      type: Object,
                      value: {
                        US: {
                           mask: '(###) ###-####',
                           maxLength: 10
                        },
                        DE: {
                           mask: '###-####'
                        }
                      }
                    },
                    label: {
                        type: String
                    },
                    value: {
                        type:String,
                        notify: true,
                        reflectToAttribute: true,
                        observer: 'formatNumber'
                    }
                }
            }

            formatNumber(newVal, oldVal) {
                var rule = this.rules[this.country];
                if (rule) {
                    var formatter = new StringMask(rule.mask);
                    if (rule.maxLength && newVal && (newVal.length > rule.maxLength)) {
                        this.value = oldVal;
                    }
                    var formattedValue = formatter.apply(this.value);
                    console.log("formatted value:"+formattedValue);
                }
            }
        }

        customElements.define(NsPhone.is, NsPhone);
    </script>
</dom-module>