Your outbound calls may drop due to the following reasons: 

• Your account could be running low on credits, please add credits under Admin>Billing>Buy Phone Credits.
• Your microphone may be disabled. To enable it, click on the lock icon in your URL address bar and select the allow option for the microphone. 
• There may be a 0 appended after the country code or you could have given the wrong country code while dialling.