Asterisk Box Behind NAT - Bad file descriptor

I hit a a brick wall.

[Jan 1 08:37:21] WARNING[126]: chan_sip.c:1787 __sip_xmit: sip_xmit of 0x24fb904 (len 468) to 208.43.85.36:5060 returned -2: Bad file descriptor
[Jan 1 08:37:21] WARNING[126]: acl.c:548 ast_ouraddrfor: Cannot connect
Reliably Transmitting (NAT) to 207.47.75.76:5060:
OPTIONS sip:207.47.75.76SIP/2.0
Via: SIP/2.0/UDP 0.0.0.0:5060;branch=z9hG4bK2a7fdb5b;rport
From: “asterisk” sip:asterisk@0.0.0.0;tag=as47055808
To: sip:207.47.75.76
Contact: sip:asterisk@0.0.0.0

I have been trying to connect a PX local behind an ADSL router to another one with a public IP for the last 6 months and I have read the documentation in here and am not making head way.

For starters the local PBX is failing to register with the public one.
I have setup externhost = dynamic dns

Version - 1.4.21.2

Do you have a route to that destination?

Have you NATted port udp/5060 from the internet to your local PBX ?

If this is possible, you may want to use SIP over TCP which will allow to perform a diagnostic about network connectivity as TCP is a connected protocol.