asterisk users - Aug 2008 - Comdial IP thru Asterisk to cell

I'm integrating an Asterisk box with our existing Comdial FX-II.  The 
Comdial has a SIP card which allows us to communicate with the Asterisk 
box, and I have set up a dialplan which does most of what I want--or 
most of what I know I want.
I have, however, tried unsuccessfully to have it ring a cell phone 
simultaneously with the SIP phones.
What I can do:
I can call from one SIP phone (no Comdial) to another SIP phone and have 
it ring a cell phone as well.  I can then answer either the cell phone 
or the SIP phone and talk.
I can call from Comdial thru the SIP card to Asterisk and then to the 
SIP phone.  I can answer the SIP phone and talk.
What I can't do:
I cannot call from Comdial thru the SIP card to Asterisk and have it 
ring both a SIP phone and a cell phone.  The cell phone will ring, but 
if I try to answer, it hangs up.
Does anyone have experience with integrating via the Comdial SIP card?
--Paul

-- 
Paul Douglas Franklin
Computer Manager, Union Gospel Mission of Yakima, Washington
Husband of Danette
Father of Laurene, Miriam, Tycko, Timothy, Sarabeth, Marie, Dawnita, Anna Leah,
Alexander, and Caleb