Call Waiting

[nareach]

I use elastix 2.4 and i have 10 Sip extensions 4 trunks

i use 4 agents to get incoming call

i just want when my 4 agents is current busy on call i want to tell caller my agent is busy please wait a moment.

and if when one of my agent free i want caller that waiting it will ring in that free agent.

sorry for my bad English.

thanks