Does not ring or notify me of call, phone goes straight to Voicemail

When i am using my phone such as streaming a youtube video, or even engaging in text messaging, my phone doesn't ring or let me know a call is coming in. I get an aiudio notification that i have a voicemail. The caller tells me (in a voicemail) that the call went straight to voicemail. When i try to call that person back, the call fails and i have to restart my phone. This happens quite frequently. I have looked through all the possible settings that can be effecting this issue to no avail. Anybody have some help with this?