Since you have a switchboard have them ask 'he's in a meeting can I take a message', have them email the message to you, then you can respond back to your staff

Since you have a switchboard have them ask 'he's in a meeting can I take a message', have them email the message to you, then you can respond back to your staff and them and have them return the call. You could also setup a block of time the following morning to return calls but I think training an assistant to handle them would free up more time for you.