Need help with gsm card readers

This is my first time working with freepbx and I was assigned to do a call center project, so here is what happened:
I installed freepbx and created extensions. I have a grandstream gxw4108 with 5 sim card readers (vodafone readers). I configured a sip trunck on both sides and when i configured the first two sim cards they worked well but when I added the rest the following thing happened:
once I dial the first call the sim card readers gets busy one by one and no more calls can be done until i restart the readers.
I can’t but any config right now. I will when i go to the site.
any ideas guys please help.
thanks in advance.