Busy landline result into missing calls (US)

So we have call forwarding setup during business hours

The sequence is

1. Landline Number (Cable service)

2. Business A cellphone #

3. Business B cellphone #

4. Business C cellphone #

Our problem is when the landline phone (1.) is in use, I'm assuming the next ring will go to Business A cell number, but it doesn't, we end up missing the call entirely. As a healthcare business, all calls are important to us. Please advise.