HTML

<input type="text" name="mobileNumber" formControlName="mobile" id="atg_store_telephoneInput" class="iDimension"
          inputmode="numeric" placeholder="Ex. (***) ***-xxxx" touched phoneNumberFormater
          [maxlength]=mobileFieldMaxlength [country]="shippingForm.controls['country'].value">
            
<select name="country" formControlName="country" (change)="clearCountryDependentFields()"
            id="billing_countryNameSelect" class="iDimension"
            (change)="setMobileFieldMaxlength(shippingForm.controls['country'].value)">
            <option value="">Select a Country</option>
            <option *ngFor="let item of countryList?.data" [value]="item.code">{{ item.displayName }}</option>
          </select>

TYPESCRIPT

mobileFieldMaxlength: string = '14';

setMobileFieldMaxlength(country: string) {
    if (country === 'US') {
      this.mobileFieldMaxlength = '14';
    } else if (country !== 'US') {
      this.mobileFieldMaxlength = 'unset';
    }
    ɵdetectChanges(this);
  }