asterisk users - Nov 2009 - Cisco 7970 SIP endless ringing...?

Anyone know what would cause an endless ringing situation?

I have a snom360 and cisco 7970 (sip 8.5.3). I have an incoming trunk
which dials both phones:

[gp710]
exten => _[*1-9].,1,Dial(SIP/line1 at cisco7970&SIP/line1 at snom360,60)
exten => _[*1-9].,n,Hangup

If a call comes in, I can answer the call on the cisco no problem. 
However if I answer on the snom360, the cisco never stops ringing.

-Dan