asterisk users - Mar 2004 - zap call being dropped after 7 seconds - SIP phone with public IP (no NAT)

My ZAP calls are being dropped after 7 seconds. The only info I can find
is:

Mar 12 14:03:08 WARNING[98311]: Maximum retries exceeded on call
0009b7d0-f2bd0023-169cc5bc-224cac05@_my7960ip_ for seqno 102 (Response)
Mar 12 14:03:11 WARNING[98311]: Maximum retries exceeded on call
0009b7d0-f2bd0023-169cc5bc-224cac05@_my7960ip_ for seqno 102 (Request)

I found a possible cause at http://www.voip-info.org/wiki-Asterisk+FAQ :
Explain the warning "Maximum retries exceeded on call ... for seqno
..."!
Most likely you have a SIP client behind NAT that is trying to communicate
with Asterisk without having the "nat=yes" setting in place in
sip.conf

My 7960 has a public IP (so, it is not behind a NAT). Every call is
dropped after 7 seconds. Is it possible that a lag/excessive bandwidth use
is causing this? The voice is clear BTW. (LAGGED (215 ms))

This * box is hosted under a large Ericsson PBX, and the FXO card is an
extension of that Ericsson. I also tried to start zapata.conf with fxo_ls,
fxo_gs and fxo_ks. All 3 failed. Only signalling=fxs_ks is able to start a
call but it is dropped after 7 seconds (average).

Any ideas?

sip.conf:

[4503]
; This is a Cisco 7960 w/ SIP v. 6.2 image
type=friend
username=4503
secret=supersecret
host=dynamic
mailbox=4503
context=from-sip
canreinvite=no
nat=no

sip show peers:
Name/username    Host                 Mask             Port     Status
4503/4503        7960public_ip   (D)  255.255.255.255  5060     LAGGED (215 ms)