[asterisk-users] GOIP GSM to SIP Gateway?

[Matt]

Hi,
Has anyone here any experiencing with linking an Asterisk PBX to a
GOIP GSM to SIP Gateway?  We've got inbound calls from the GSM network
working properly, however, outbound calls seem to randomly choose a
SIM line to use.

Is there anyway (short of defining dial an 8 from this phone for this
trunk to this SIM and a 9 from this phone for a trunk to this SIM) to
get it to use certain SIM cards when calls are made from certain
phones?