In v5.1.0 we added a fix that should resolve this problem.

We don't have update mechanism for dynamic inputs. We will have to look into this. As a workaround you can achieve desired effect with *ngIf directive and multiple inputs:

import { Component, OnInit } from '@angular/core';
import { FormBuilder } from '@angular/forms';

@Component({
  selector: 'app-input-mask',
  templateUrl: './input-mask.component.html',
  styleUrls: ['./input-mask.component.scss'],
})
export class InputMaskComponent implements OnInit {
  constructor(private fb: FormBuilder) {}

  form = this.fb.group({
    phone: '',
  });
  ngOnInit(): void {}
  phoneMask: string;
  changePhoneMask(country: any) {
    switch (country) {
      case 'USA':
      case 'CAN':
        this.phoneMask = '+1 (999) 999-9999 Ext: 9999';
        break;
      case 'GBR':
        this.phoneMask = '+44 999 999 9999';
        break;
    }
  }
}

and template:

  <div class="input-group">
      <div class="input-group-prepend">
        <mdb-select (selected)="changePhoneMask($event)">
          <mdb-option [value]="'USA'">United States</mdb-option>
          <mdb-option [value]="'CAN'">Canada</mdb-option>
          <mdb-option [value]="'GBR'">United Kingdom</mdb-option>
        </mdb-select>
      </div>
      <form [formGroup]="form">
        <input
          *ngIf="!phoneMask"
          mdbInputMask=""
          [clearIncomplete]="false"
          type="tel"
          class="form-control"
          formControlName="phone"
        />
        <input
          *ngIf="phoneMask === '+1 (999) 999-9999 Ext: 9999'"
          mdbInputMask="{{ phoneMask }}"
          [clearIncomplete]="false"
          type="tel"
          class="form-control"
          formControlName="phone"
        />
        <input
          *ngIf="phoneMask === '+44 999 999 9999'"
          mdbInputMask="{{ phoneMask }}"
          [clearIncomplete]="false"
          type="tel"
          class="form-control"
          formControlName="phone"
        />
      </form>
    </div>