Immediately forward the call to voicemail. The best thing to do in this situation is immediately forward the call to voicemail. Unless you are waiting for an important work-related call, most calls can wait until you are done with the customer.
Just letting the phone continue ringing is distracting. quickly answering the phone saying, I am busy right now, I willcall you right back is sometimes appropriate, but it distracts from the conversation with your customer and should be avoided if possible.