I have a freeswitch based PBX that has been working fine. I was using Skype connect as a SIP provider and I have had no difficulty making and receiving calls using this. Also, no difficulty with internal local-local calls.
I have just changed my sip trunk provider to voip-unlimited (based in the UK) and updated my sip profile accordingly. I can receive calls fine with the new provider, but when I make a call, the other party can hear me, but I cannot hear them. I do not get a ringing tone when I dial out (the remote party's phone rings, he answers the call, he hears me, but I cannot hear him).
I have ports 5060 and 5080 open to both UDP and TCP traffic and the router also supports PnP. I am uncertain if it is a firewall issue but certainly no problems were experienced with Skype connect previously.
the best thing would be to run a packet sniffer (tcpdump or wireshark) and see what's going on when the call is set up.
It might be: