VOIP Not working with Juniper firewall

Hi Friends,
We have Juniper Firewall connected with Grandstream GXP1400 and also linksys FXS, when we connect, works for some time then experience one way voice and not getting incoming.
can some one suggest the configuration in the juniper or in voip?
Thanks in advance.

For one thing turn of SIP ALG if it has one.

We also need a bit more info to help. Tell us a little more about the network and the FreePBX server.