Caller can hear me clearly, but I cannot hear the caller

[jptg5Y]

In my VoXaLot account, I did Call Forwarding to:
sip:12345@myAsteriskServer.com

Where 12345 is an extension on my Asterisk server.

Now I can get the phone at (my Asterisk) extension
12345 to ring when there is an incoming call on my
12345@voxalot.com account.

My Asterisk server is behind a firewall.
I added 2 new rules to my firewall:
1. Inbound UDP traffic on port 5060 should be forwarded to the Asterisk server at 192.168.6.100 at port 5060
2. This inbound UDP traffic is limited to the 64.34.173.199 ip address which is premium.voxalot.com


So now:
A. My friend calls a local PSTN number from http://www.sipbroker.com/sipbroker/action/pstnNumbers
B. He enters my ENUM
C. (Given my voxalot call forwarding) I get UDP traffic on port 5060 from 64.34.173.199
D. My asterisk 12345 extension starts ringing

Good. So far.

We start talking.
He can hear my voice very clearly.
But the problem is - I cannot hear his voice at all. Absolutely nothing.

I suspect this could be a firewall issue.
Yes, my asterisk server is behind my firewall's NAT.

Do I need to open other ports and/or allow additional ip addresses?
Or is this some other problem?