Your phone or device is probably not registered, you can check your account status on our website after logging in. If your phone is not showing up as online you may have problems making or receiving calls. Please follow the steps in 'I cannot connect to VoIPon'.
Also, check what codecs you are using and whether they need licensed or purchasable software to work.
For example, the G729 codecs require licenses to be used.
For outgoing calls, make sure you have credit on your account or are subscribed to a relevant tariff. Also, make sure that the number you are calling is not a mobile number if you cannot dial it from your Unlimited account and have no call credit.