asterisk users - Feb 2007 - troubles gsm-gateway no free channels

hi list

i'm struggling with my new voiceblue gateway an my asterisk...

i've got three simcards from the same provider, all in one voip-gsm- 
gateway. how can i tell my asterisk, that the 4th call, to this  
provider, can not be routed through the gateway but to another trunk?

asterisk log when trying the 4th call:

-- Got SIP response 503 "Service Unavailable" back from 10.19.0.53  
<-- of course... only three sim-cards
     -- SIP/10.19.0.53-08221730 is circuit-busy
   == Everyone is busy/congested at this time (1:0/1/0)

thanks in advance for any ideas!

best regards matthias