Setting up a phone hunt group for a major event?

I work for a large nonprofit Walk a thon. Every year, we set up a nerve center in a local room near Boston Common. This room houses staff members who answer calls from volunteers along the route and field their questions and concerns.

It requires us to have Verizon set up a hunt group of 5 different phone lines every year at that location.

Several issues we’ve had. We use a 20 year old AT&T splitter that only works with compatible phones.
Last year, we lost one of the phones and was forced to try and figure out a solution but really couldn’t solve it so one line was silent.

Apart from this, it works funny. The phones don’t ring at the same time. Only the first phone rings and if that’s busy it bounces to the next one and so on. This means that whoever has that first phone is always getting calls.

Perfect scenario: We want to give one number out to all of our volunteers that they can call if they need information or to report something.
On the other end of that phone number is 5 phones that ring simultaneously until someone answers it.