Retransmission timeout error - SIP NAT issue

when you have this problem in the /var/log/asterisk/full as below .....

[May 15 15:29:09] WARNING[2376] chan_sip.c: Retransmission timeout reached on transmission ZTlmMDZlNjk1ODVkNWU5ZGRlNmI2ZWFhZGM5MDU5NGY. for seqno 2 (Critical Response) -- See https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions
Packet timed out after 6401ms with no response
[May 15 15:29:09] WARNING[2376] chan_sip.c: Hanging up call ZTlmMDZlNjk1ODVkNWU5ZGRlNmI2ZWFhZGM5MDU5NGY. - no reply to our critical packet (see https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions).
[May 15 15:29:09] VERBOSE[2420][C-00000000] res_agi.c:     -- AGI Script a2billing.php completed, returning 4
[May 15 15:29:09] VERBOSE[2420][C-00000000] pbx.c:   == Spawn extension (PSTNOutBound, 60383215500, 9) exited non-zero on 'SIP/5106-00000000'

It was due to /var/www/html/admin/modules/core/etc/sip.conf
externip= (MUST SET TO THE CORRECT EXTERNAL IP Address)