ACK Problem : Not routed to NAT'd port

Hello list.
I'm facing a problem with my SER configuration and a call to a NAT'd
endpoint. When I call a NAT'd endpoint registered in my SER proxy, the
call is established, but the last ACK message is not properly routed by SER.
You can see this in the next messsage flow.

100.100.100.50 : Callee
100.100.100.99 : SER
100.100.100.231 : Called

U 100.100.100.50:5060 -> 100.100.100.99:5060
INVITE sip:008991000404@sip.mydomain.com SIP/2.0.
Via: SIP/2.0/UDP
100.100.100.50;rport;branch=z9hG4bK404c94810000005a43b2b7ee000053dc0000010f.
Contact: <sip:551100000121@100.100.100.50;user=phone>.

U 100.100.100.99:5060 -> 100.100.100.50:5060
SIP/2.0 100 trying -- your call is important to us.
Via: SIP/2.0/UDP
100.100.100.50;rport=5060;branch=z9hG4bK404c94810000005a43b2b7ee000053dc0000
010f.

U 100.100.100.99:5060 -> 100.100.100.231:62447
INVITE sip:551100000684@100.100.100.231;user=phone SIP/2.0.
Record-Route: <sip:100.100.100.99;ftag=979715625143;lr=on>.
Via: SIP/2.0/UDP 100.100.100.99;branch=z9hG4bKe4c7.150a3f81.0.
Via: SIP/2.0/UDP
100.100.100.50;rport=5060;branch=z9hG4bK404c94810000005a43b2b7ee000053dc0000
010f.
Contact: <sip:551100000121@100.100.100.50;user=phone>.

U 100.100.100.231:62447 -> 100.100.100.99:5060
SIP/2.0 100 Trying.
Via: SIP/2.0/UDP 100.100.100.99;branch=z9hG4bKe4c7.150a3f81.0,SIP/2.0/UDP
100.100.100.50;rport=5060;branch=z9hG4bK404c94810000005a43b2b7ee000053dc0000
010f.

(The call is answered)

U 100.100.100.231:62447 -> 100.100.100.99:5060
SIP/2.0 200 OK.
Via: SIP/2.0/UDP 100.100.100.99;branch=z9hG4bKe4c7.150a3f81.0,SIP/2.0/UDP
100.100.100.50;rport=5060;branch=z9hG4bK404c94810000005a43b2b7ee000053dc0000
010f.
Contact: <sip:551100000684@100.100.100.231;user=phone>.

U 100.100.100.99:5060 -> 100.100.100.50:5060
SIP/2.0 200 OK.
Via: SIP/2.0/UDP
100.100.100.50;rport=5060;branch=z9hG4bK404c94810000005a43b2b7ee000053dc0000
010f.
Contact: <sip:551100000684@100.100.100.231;user=phone>.

U 100.100.100.50:5060 -> 100.100.100.99:5060
ACK sip:551100000684@100.100.100.231;user=phone SIP/2.0.
Via: SIP/2.0/UDP
100.100.100.50;rport;branch=z9hG4bK404c94810000005a43b2b7f00000182a00000112.
Route: <sip:100.100.100.99;ftag=979715625143;lr=on>.

U 100.100.100.99:5060 -> 100.100.100.231:5060
ACK sip:551100000684@100.100.100.231;user=phone SIP/2.0.
Record-Route: <sip:100.100.100.99;ftag=979715625143;lr=on>.
Via: SIP/2.0/UDP 100.100.100.99;branch=0.
Via: SIP/2.0/UDP
100.100.100.50;rport=5060;branch=z9hG4bK404c94810000005a43b2b7f00000182a0000
0112.

U 100.100.100.50:5060 -> 100.100.100.99:5060
ACK sip:551100000684@100.100.100.231;user=phone SIP/2.0.
Via: SIP/2.0/UDP
100.100.100.50;rport;branch=z9hG4bK404c94810000005a43b2b7f000001d6e00000114.
Route: <sip:100.100.100.99;ftag=979715625143;lr=on>.
.

(.........)

(Timeout and the call is released)

U 100.100.100.231:62447 -> 100.100.100.99:5060
BYE sip:551100000121@100.100.100.50;user=phone SIP/2.0.
Via: SIP/2.0/UDP
192.168.1.20;rport;branch=z9hG4bKc0a801140000002643b2a9be000036e10000001f.

Route: <sip:100.100.100.99;ftag=979715625143;lr=on>.
.

U 100.100.100.99:5060 -> 100.100.100.50:5060
BYE sip:551100000121@100.100.100.50;user=phone SIP/2.0.
Record-Route: <sip:100.100.100.99;ftag=93794163011;lr=on>.
Via: SIP/2.0/UDP 100.100.100.99;branch=z9hG4bK15c7.924c3535.0.
Via: SIP/2.0/UDP
192.168.1.20;received=100.100.100.231;rport=62447;branch=z9hG4bKc0a801140000
002643b2a9be000036e10000001f.

U 100.100.100.50:5060 -> 100.100.100.99:5060
SIP/2.0 200 OK.
Via: SIP/2.0/UDP 100.100.100.99;branch=z9hG4bK15c7.924c3535.0,SIP/2.0/UDP
192.168.1.20;rport=62447;received=100.100.100.231;branch=z9hG4bKc0a801140000
002643b2a9be000036e10000001f.

U 100.100.100.99:5060 -> 100.100.100.231:62447
SIP/2.0 200 OK.
Via: SIP/2.0/UDP
192.168.1.20;rport=62447;received=100.100.100.231;branch=z9hG4bKc0a801140000
002643b2a9be000036e10000001f.

I don't understand why the the ACK message is routed to port 5060 and not to
the port 62447 like the INVITE or the final OK. Is this maybe a
configuration problem?.
My ser.cfg is based on the onsip.org configuration documents plus some
modifications by me.

Can someone help me here
Thanks in advance
Ricardo Martinez.-