GoIP4 gsm gateway and freepbx trunk


I’m a newbie to voip and could anyone suggest me on below scenario?
i have goip4 gsm gateway with 4 sim channels + freepbx for gsm outgoing call termination.
Goip is in trunk gateway mode with single trunk to freepbx. Goip is not registered with freepbx, only outbound setting configured in trunk. Any inbound call to freepbx will lead to ivr and then go out from the trunk to goip gateway. The 1st call goes out from 1st gsm channel successfully, but the second call onwards fails. Do i need to create additional trunks for concurrent calls? What is maximum channel in freepbx trunk setting? Or is it something to do with goip gateway setting alone? (such as round robin outbound call setting)

here is the trunk setting;

peer detail:
host= goip ip address
port= 5060 (goip port)
allow= ulaw&alaw