Please check that you do not have the Do Not Disturb (DND) function enabled on the phone.  If not and the problem still persists please check the ringer volume on the phone to make sure it is turned up. 

If this is set correctly then the next step would be to unplug the phone, completely disconnecting it from the power supply and network and then after 5 minutes re-connect the phone again so that it powers up and re-establishes a network connection.  This reboot should then resolve the issue. 

If not please contact our Support Helpdesk for assistance.