There could be several reasons. Firstly, check your account has been activated by the LimePhone team.

Check your internet connection (cellular data or Wi-Fi) as LimePhone relies on internet for calls. Ensure you have enough calling minutes credits in your plan and haven't reached any limits. Verify the dialed number is correct and doesn't contain typos.

Verify if your account has enough credits as calls outside USA are billed per minute. Finally, restart the LimePhone app and your device.