Call Queue That Calls Out To Agent For The First Caller. I *think* this works, but I'm not a Rubyist so may have some n00b gaffes.

app.rb

require 'rubygems'
require 'sinatra'
require 'twilio-ruby'

# Endpoint for the call-in line that your customers call.
post '/caller' do
  Twilio::TwiML::Response.new do |r|
    r.Enqueue 'Customer Queue', :waitUrl => '/wait'
  end.text
end

# Custom Wait Room that will dequeue the user if it is the first one to call the agent.
post '/wait' do
  Twilio::TwiML::Response.new do |r|
    if params[:QueuePosition] == "1"
      r.Leave
    else
      r.Play "http://yourserver.com/yourmusic.mp3"
    end
  end.text
end

# Call the agent
post '/callagent' do
  Twilio::TwiML::Response.new do |r|
    r.Dial do |d|
      r.Number "+15556667777" ### Your agent's number, could be dynamically generated.
    end
  end
end