Generally when I spot a call coming in on my cellphone, providing I have the phone within my sight because I wear a bluetooth most of the day, I often will send a custom templated text message to the caller if I am not available to answer the call. The reason I would be unable to answer the call is most likely because I am already on a call. I generally don't hang up and grab another call unless I'm about ready to hang up. Otherwise, I keep talking because my business at hand is important.
But I have met agents who will hang up. If I am expecting an important telephone call, like a person I have a super hard time getting on the phone, for example, I will tell the person when I call that I might have to hang up if that particular person calls. Otherwise, all calls go to voice mail and / or receive a text message when I'm on the phone.
You can read more about this pecular practice in my personal blog today at this link: A True Story About Setting Priorities for the Self Employed.