Phone call pausing help

I went back to do some more troubleshooting. We only have 5 ports that we pay for. I was able to replicate this issue. If 5 calls are active, and a 6th call comes in, every time that 6th call rings the phones the calls will drop AUDIO. Once that 6th call stops ringing the issue goes away and calls work fine. Is there a way to send that 6th call to Voicemail or possibly not ring the phones and leave in a queue until a call drops?