asterisk users - Nov 2003 - distinctive ring doesn't work

In my extensions.conf file I'm attempting to distinctively ring one of my 
zap channels with a different ring depending upon whether the call is 
received from the DID or inside extension.  The DID extension looks like 
so:
	exten => 5551236543,1,Dial,Zap/28r1|20

However, when I dial in on the DID the phone is not ringing at all.  The 
CLI shows that Zap/28 is ringing and you can hear it ringing on the 
outside line you call from but you can't hear it on the actual phone that 
is being dialed.  It then goes to voicemail.

Can anyone tell me what I am doing wrong here?  
Thanks
AJ