In order to make the call you need the number to be in the international dialing format.
In some cases the dialer will stay grayed out if the format is not recognized. Please make sure the format of the number is correct.
You can check the characters and format.
If you see any other message you can try restarting the browser to check if it was cache related.
For any questions regarding the dialer and our voice features feel free to share it with the support team.