Multiple Call Issues and Voicemail Situation

Haha… the situation is so bad it’s almost funny. Right now, we have a set main line, and multiple people are logged in to it. We’re a small five person company, and have everyone logged into the same line, so that we can all access the voicemails and messages. If we use extensions, then everyone can only see their own individual extension and not the main voicemails and messages. We do not have a front desk administrator or secretary so we can’t have it where everyone cannot see it. I figured it was possible it was the number of phones connected, but this wasn’t a problem before, only recently became a problem. This is a huge issue because recently, when our clients call in, if we’re already on a call, the incoming call automatically mutes both us and the other person, meaning they can’t hear us and we can’t hear them, and we have to hang up on the incoming call to get back to the first one. In the meantime, the first call sometimes immediately gets disconnected. Since this issue started, we’ve had two new issues pop up, which I’m including in case they’re all related and my team messed up somewhere. The first new issue is the one isn’t ringing consistently. People are getting sent directly to voicemail even though people are available to answer the phones, none of the phones or the ring system are set to do not disturb,, and it is inside our set office hours. When we do get calls, sometimes it disconnects before we can even say hello. We have a steady internet signal, and all apps and phones are completely updated. A new issue has even popped up - one of my team member’s lines is getting all the voicemails instead of the main line, so the main voicemails don’t even pop up in the main box anymore - the voicemails go to her voicemail on her phone where no one but her can see them. Does anyone know what’s going on? Are we missing something major or just done something stupid? It’s really effecting our business and we don’t know how to fix it. And we’re extremely desperate to fix it. Does anyone have any ideas?