If your call diversions are all set to Voicemail then this can happen. Please dial this code **004*321# to activate diversions for Busy, No Reply and Unreachable calls. Once this is done your phone will ring and not straight go to voicemail.
Please refer to our Email Help & Support page for further assistance. If the issue still continues then you are best to speak with our Technical Support team via the messaging service to assist you further.