asterisk users - Sep 2005 - QUESTION: RINGING CONTINUES DURING CALL

After searching around, I've been unable to to find any relevant info on 
this.  Perhaps the group can help?

I am seeing something strange with a new Sipura SPA-3000 (and I've 
noticed this also with an IAX softphone):

When I dial 777, this dialplan (in extensions.conf) is run:

   exten => 777,1,Dial(Zap/1/2345678)
   exten => 777,n,Hangup

The number is answered by the called party, but the ringing sound 
continues and is heard over top of the conversation.

If I add an Answer line to the dialplan, this problem disappears:

   exten => 777,1,Answer
   exten => 777,n,Dial(Zap/1/2345678)
   exten => 777,n,Hangup

This does not occur with a Grandstream BT-101 or an XTen SIP softphone.

Why do some devices need the "Answer" line in the dialplan before the 
"Dial" line?  (I'd rather not have to do a custom section for
certain
devices).

Thanks for any info...

Alvin