Bob using Real IP , Armando Private IP behind NAT.... rtpproxy is enforced.
Call 1 = From Bob -> Armando = Success
Call 2 = From Armando -> Bob = Failed because Armando UA never reply with ACK after 200 Ok Message
############

Call 1 caller: Bob , Callee : Armando.... Successfull Call Setup

U Armando -> SER
  SIP/2.0 200 OK..Via: SIP/2.0/UDP 10.1.90.116;branch=z9hG4bK1ba9.f6fe0847.0..Via: SIP/2.0/UDP 66.231.238.215:5060..Record-
  Route: <sip:3056034425@10.1.90.116;ftag=1564176202;lr=on>..From: <sip:1234@10.1.90.116;user=phone>;tag=1564176202..To: <s
  ip:3056034425@10.1.90.116;user=phone>;tag=642782702..Call-ID: 2352639743@66.231.238.215..CSeq: 1 INVITE..Contact: <sip:30
  56034425@192.168.0.103:5061;user=phone;transport=udp>..Server: Cisco ATA 186  v2.16 ata18x (030509a)..Content-Length: 203
  ..Content-Type: application/sdp....v=0..o=3056034425 62077 62077 IN IP4 192.168.0.103..s=ATA186 Call..c=IN IP4 192.168.0.
  103..t=0 0..m=audio 16384 RTP/AVP 0 101..a=rtpmap:0 PCMU/8000/1..a=rtpmap:101 telephone-event/8000..a=fmtp:101 0-15..
#
U SER -> Bob
  SIP/2.0 200 OK..Via: SIP/2.0/UDP 66.231.238.215:5060..Record-Route: <sip:3056034425@10.1.90.116;ftag=1564176202;lr=on>..F
  rom: <sip:1234@10.1.90.116;user=phone>;tag=1564176202..To: <sip:3056034425@10.1.90.116;user=phone>;tag=642782702..Call-ID
  : 2352639743@66.231.238.215..CSeq: 1 INVITE..Contact: <sip:3056034425@66.176.127.120:5061;user=phone;transport=udp>..Serv
  er: Cisco ATA 186  v2.16 ata18x (030509a)..Content-Length: 242..Content-Type: application/sdp....v=0..o=3056034425 62077
  62077 IN IP4 192.168.0.103..s=ATA186 Call..c=IN IP4 66.231.238.115..t=0 0..m=audio 35118 RTP/AVP 0 101..a=rtpmap:0 PCMU/8
  000/1..a=rtpmap:101 telephone-event/8000..a=fmtp:101 0-15..a=direction:active..a=nortpproxy:yes..
#
U Bob -> SER
  ACK sip:3056034425@10.1.90.116 SIP/2.0..Route: <sip:3056034425@66.176.127.120:5061;user=phone;transport=udp>..Via: SIP/2.
  0/UDP 66.231.238.215:5060..From: <sip:1234@10.1.90.116;user=phone>;tag=1564176202..To: <sip:3056034425@10.1.90.116;user=p
  hone>;tag=642782702..Call-ID: 2352639743@66.231.238.215..CSeq: 1 ACK..User-Agent: Cisco ATA 186  v2.15 ata18x (030313a)..
  Content-Length: 0....
#
U SER -> Armando
  ACK sip:3056034425@66.176.127.120:5061;user=phone;transport=udp SIP/2.0..Max-Forwards: 10..Record-Route: <sip:3056034425@
  10.1.90.116;ftag=1564176202;lr=on>..Via: SIP/2.0/UDP 10.1.90.116;branch=0..Via: SIP/2.0/UDP 66.231.238.215:5060..From: <s
  ip:1234@10.1.90.116;user=phone>;tag=1564176202..To: <sip:3056034425@10.1.90.116;user=phone>;tag=642782702..Call-ID: 23526
  39743@66.231.238.215..CSeq: 1 ACK..User-Agent: Cisco ATA 186  v2.15 ata18x (030313a)..Content-Length: 0....
##################

Call 2 caller: Armando , Callee : Bob.... Failed Call Setup .... 

U Bob -> SER
  SIP/2.0 200 OK..Via: SIP/2.0/UDP 10.1.90.116;branch=z9hG4bK4544.49cf0507.0..Via: SIP/2.0/UDP 192.168.0.103:5061;received=
  66.176.127.120..Record-Route: <sip:1234@10.1.90.116;ftag=298050148;lr=on>..From: <sip:3056034425@lserver.telecode.com;use
  r=phone>;tag=298050148..To: <sip:1234@lserver.telecode.com;user=phone>;tag=1758946682..Call-ID: 2670895917@192.168.0.103.
  .CSeq: 1 INVITE..Contact: <sip:1234@66.231.238.215:5060;user=phone;transport=udp>..Server: Cisco ATA 186  v2.15 ata18x (0
  30313a)..Content-Length: 197..Content-Type: application/sdp....v=0..o=1234 1415 1415 IN IP4 66.231.238.215..s=ATA186 Call
  ..c=IN IP4 66.231.238.215..t=0 0..m=audio 16384 RTP/AVP 0 101..a=rtpmap:0 PCMU/8000/1..a=rtpmap:101 telephone-event/8000.
  .a=fmtp:101 0-15..
#
U SER -> Armando
  SIP/2.0 200 OK..Via: SIP/2.0/UDP 192.168.0.103:5061;received=66.176.127.120..Record-Route: <sip:1234@10.1.90.116;ftag=298
  050148;lr=on>..From: <sip:3056034425@lserver.telecode.com;user=phone>;tag=298050148..To: <sip:1234@lserver.telecode.com;u
  ser=phone>;tag=1758946682..Call-ID: 2670895917@192.168.0.103..CSeq: 1 INVITE..Contact: <sip:1234@66.231.238.215:5060;user
  =phone;transport=udp>..Server: Cisco ATA 186  v2.15 ata18x (030313a)..Content-Length: 235..Content-Type: application/sdp.
  ...v=0..o=1234 1415 1415 IN IP4 66.231.238.215..s=ATA186 Call..c=IN IP4 66.231.238.115..t=0 0..m=audio 35122 RTP/AVP 0 10
  1..a=rtpmap:0 PCMU/8000/1..a=rtpmap:101 telephone-event/8000..a=fmtp:101 0-15..a=direction:active..a=nortpproxy:yes..
#

------ no ACK send back....in this moment Armando UA just open rtp and start sending voice streams
------ Bob User Agent resend this several times , but never get ACK message from Armando UA
U Bob -> SER
  SIP/2.0 200 OK..Via: SIP/2.0/UDP 10.1.90.116;branch=z9hG4bK4544.49cf0507.0..Via: SIP/2.0/UDP 192.168.0.103:5061;received=
  66.176.127.120..Record-Route: <sip:1234@10.1.90.116;ftag=298050148;lr=on>..From: <sip:3056034425@lserver.telecode.com;use
  r=phone>;tag=298050148..To: <sip:1234@lserver.telecode.com;user=phone>;tag=1758946682..Call-ID: 2670895917@192.168.0.103.
  .CSeq: 1 INVITE..Contact: <sip:1234@66.231.238.215:5060;user=phone;transport=udp>..Server: Cisco ATA 186  v2.15 ata18x (0
  30313a)..Content-Length: 197..Content-Type: application/sdp....v=0..o=1234 1415 1415 IN IP4 66.231.238.215..s=ATA186 Call
  ..c=IN IP4 66.231.238.215..t=0 0..m=audio 16384 RTP/AVP 0 101..a=rtpmap:0 PCMU/8000/1..a=rtpmap:101 telephone-event/8000.
  .a=fmtp:101 0-15..
#
U SER -> Armando
  SIP/2.0 200 OK..Via: SIP/2.0/UDP 192.168.0.103:5061;received=66.176.127.120..Record-Route: <sip:1234@10.1.90.116;ftag=298
  050148;lr=on>..From: <sip:3056034425@lserver.telecode.com;user=phone>;tag=298050148..To: <sip:1234@lserver.telecode.com;u
  ser=phone>;tag=1758946682..Call-ID: 2670895917@192.168.0.103..CSeq: 1 INVITE..Contact: <sip:1234@66.231.238.215:5060;user
  =phone;transport=udp>..Server: Cisco ATA 186  v2.15 ata18x (030313a)..Content-Length: 235..Content-Type: application/sdp.
  ...v=0..o=1234 1415 1415 IN IP4 66.231.238.215..s=ATA186 Call..c=IN IP4 66.231.238.115..t=0 0..m=audio 35122 RTP/AVP 0 10
  1..a=rtpmap:0 PCMU/8000/1..a=rtpmap:101 telephone-event/8000..a=fmtp:101 0-15..a=direction:active..a=nortpproxy:yes..
#
U Bob -> SER
  SIP/2.0 200 OK..Via: SIP/2.0/UDP 10.1.90.116;branch=z9hG4bK4544.49cf0507.0..Via: SIP/2.0/UDP 192.168.0.103:5061;received=
                                                                                                            138,3         56%