asterisk users - Aug 2005 - Calls not cleared down if extra destinations or dial commands added to extension

We have a weird situation where if the external called hangs up the call
before it is answered asterisk seems not to handle it if the original
dial command is replaced following a timeout. 

We are trying to pass the call to the main reception, but if there is no
answer then it should ring another extension in addition to the first
extension the idea being that we don't end up with people chasing the
call from desk to desk. 

In the example below if the caller hangs up during step 4 asterisk will
continue on to step 5 and start recording a voicemail.

I can't see that we we are trying to do anything unusual here, anyone
able to shed any light?

exten => 470,1,SetCIDName("Tech Support")
exten => 470,2,Dial(SIP/1,10,tr)
exten => 470,3,Dial(SIP/1&SIP/2,10,tr)
exten => 470,4,Dial(SIP/1&SIP/2&SIP/23,10,tr)
exten => 470,5,Voicemail(sb000)