[Asterisk-Users] How to get call back when transfer fails

[Andrew Kohlsmith]

I searched the 22490 messages I have in my own personal asterisk-users archive 
and have not found the answer, and it also does not appear on the wiki.

I have a SIP phone and a regular phone on a TDM400P FXS interface.  Extensions 
are 100 and 101, respectively.

On the SIP phone I can hit #, get the "Transfer" prompt and enter an extension 
I want to transfer to.  No problem.  I can do the same thing on the FXS port.

My question is does anyone have a dialplan that will bring the call I 
transfered back to me if the transfer fails (i.e. busy extension or an 
extesion that does not answer)?  I see many examples of going to voicemail 
but I have no idea how to get the call back to me.

The general idea: 
- I call someone or I receive a call.  
- I want to transfer said call, so I hit # and enter the extension
- if the extension answers, it's all good.
- if the extension is busy or does not answer, give me the call again.  

The dialplan is pretty straightforward:
exten = 100,1,Dial(SIP/224,10,t)
exten = 100,2,Congestion
exten = 101,1,Dial(Zap/1,10,t)
exten = 101,2,Congestion

I do not wish to have the transferred call go to voicemail.  I want the call 
back.  I didn't see any kind of application which would connect me back to 
the call, and simply dropping off the dialplan just hangs up.  

So far the only thing I can think of is to park the call but that is 
suboptimal. 

Anyone?

-A.