You can try the phone activation method provided from Microsoft.
Please try the following steps:
When the customer wants instructions on how to do this, please give the following steps:
1. Run the activation script: [Activation.ps1] once.
2. Type "slui 04" in the Run prompt (windows key + R)
3. Choose the appropriate country from the pulldown.
4. Dial the phone number given in the next page.
5. Follow the instructions given in order to activate license.
When further assistance is needed with phone activation, please contact OS vendor support.