[Serusers] local ACK found -> dropping it!

Having some troubles with a new voip gateway service we're using; hoping someone could provide any advice. To sum up: Call is initiated from pots line; gets routed to voip gateway provider; gateway forwards invite to our ser server; one ring then busy signal is heard on originating/requesting pots line; the destined softphone UA, registered via our ser server, never gets call. As far as I can fathom, ser recieves an INVITE forwarded/routed from the gateway, then ser requests auth/cred, then gateway responds with an ACK and at that point everything just stops. Assistance is much appreciated, as I'm unable to determine what may be causing the issue. 208.xxx.xxx.xx = 3rd-party/commercial voip gateway server 206.xx.xx.x = our ser proxy server 1234567890 = originating pots line 8889997777 = softphone UA number ( aliased to sip uri &#39;blah(a)206.xx.xx.x&#39; ) ngrep: # U 208.xxx.xxx.xx:5060 -> 206.xx.xx.x:5060 INVITE sip:8889997777@206.xx.xx.x SIP/2.0..Via: SIP/2.0/UDP 208.xxx.xxx.xx:5060;branch=z9hG4bK40f0830c;rport..From: "John Doe" <sip:1234567890@208.xxx.xxx.xx>;tag=as5f35b551..To: <s ip:8889997777@206.xx.xx.x>..Contact: <sip:1234567890@208.xxx.xxx.xx>..Call-ID: 02c132b659e84c2321debbf226051e3d@208.xxx.xxx.xx..CSeq: 102 INVITE..User-Agent: Asterisk PBX..Max-Forwards: 70 ..Date: Fri, 24 Feb 2006 22:28:05 GMT..Allow: INVITE, ACK, CANCEL, OPTIONS, BYE, REFER, SUBSCRIBE, NOTIFY..Content-Type: application/sdp..Content-Length: 397....v=0..o=root 32669 32669 IN IP4 208.xxx.xxx.xx..s=session..c=IN IP4 208.xxx.xxx.xx..t=0 0..m=audio 10784 RTP/AVP 0 18 8 3 111 4 97 101..a=rtpmap:0 PCMU/8000..a=rtpmap:18 G729/8000..a=fmtp:18 annexb=no..a=rtpmap:8 PCMA/80 00..a=rtpmap:3 GSM/8000..a=rtpmap:111 G726-32/8000..a=rtpmap:4 G723/8000..a=rtpmap:97 iLBC/8000..a=rtpmap:101 telephone-event/8000..a=fmtp:101 0-16..a=silenceSupp:off - - - -.. # U 206.xx.xx.x:5060 -> 208.xxx.xxx.xx:5060 SIP/2.0 407 Proxy Authentication Required..Via: SIP/2.0/UDP 208.xxx.xxx.xx:5060;branch=z9hG4bK40f0830c;rport=5060..From: "John Doe" <sip:1234567890@208.xxx.xxx.xx>;tag=as5f35b551..To : <sip:8889997777@206.xx.xx.x>;tag=2d38da4c38ea547b795405b32554c3bb.f3cb..Call-ID: 02c132b659e84c2321debbf226051e3d@208.xxx.xxx.xx..CSeq: 102 INVITE..Proxy-Authenticate: Digest realm="208.1 39.204.245", nonce="43fe339b33d7c58a0d89e4711838584d"..Server: Sip EXpress router (0.9.4 (i386/linux))..Content-Length: 0..Warning: 392 206.xx.xx.x:5060 "Noisy feedback tells: pid=2 974 req_src_ip=208.xxx.xxx.xx req_src_port=5060 in_uri=sip:8889997777@206.xx.xx.x out_uri=sip:8889997777@206.xx.xx.x via_cnt==1".... # U 208.xxx.xxx.xx:5060 -> 206.xx.xx.x:5060 ACK sip:8889997777@206.xx.xx.x SIP/2.0..Via: SIP/2.0/UDP 208.xxx.xxx.xx:5060;branch=z9hG4bK40f0830c;rport..From: "John Doe" <sip:1234567890@208.xxx.xxx.xx>;tag=as5f35b551..To: <sip: 8889997777@206.xx.xx.x>;tag=2d38da4c38ea54795405554c3bb...Contact: <sip:1234567890@208.xxx.xxx.xx>..Call-ID: 02c132b659e84c2321debbf226051e3d@208.xxx.xxx.xx..CSeq: 102 ACK..User-A gent: Asterisk PBX..Max-Forwards: 70..Content-Length: 0.... ... end of transmission; no further exchange after gateway ACK; originating pots line hears a single ring, then fast busy signal; softphone UA never recieves INVITE or anything ser debug: IP Request: 0(2974) method: <INVITE> 0(2974) uri: <sip:8889997777@206.xx.xx.x> 0(2974) version: <SIP/2.0> 0(2974) parse_headers: flags=1 0(2974) Found param type 232, <branch> = <z9hG4bK40f0830c>; state=6 0(2974) Found param type 235, <rport> = <n/a>; state=17 0(2974) end of header reached, state=5 0(2974) parse_headers: Via found, flags=1 0(2974) parse_headers: this is the first via 0(2974) After parse_msg... 0(2974) preparing to run routing scripts... 0(2974) parse_headers: flags=128 0(2974) end of header reached, state=9 0(2974) DEBUG: get_hdr_field: <To> [31]; uri=[sip:8889997777@206.xx.xx.x] 0(2974) DEBUG: to body [<sip:8889997777@206.xx.xx.x> ] 0(2974) get_hdr_field: cseq <CSeq>: <102> <INVITE> 0(2974) DEBUG:maxfwd:is_maxfwd_present: value = 70 0(2974) DBG:maxfwd:process_maxfwd_header: value 70 decreased to 16 0(2974) DEBUG: add_param: tag=as5f35b551 0(2974) end of header reached, state=29 0(2974) parse_headers: flags=256 0(2974) DEBUG: get_hdr_body : content_length=397 0(2974) found end of header 0(2974) find_first_route: No Route headers found 0(2974) loose_route: There is no Route HF 0(2974) is_local(): Realm '206.xx.xx.x' is local 0(2974) parse_headers: flags=16384 0(2974) pre_auth(): Credentials with given realm not found 0(2974) build_auth_hf(): 'Proxy-Authenticate: Digest realm="208.xxx.xxx.xx", nonce="43fe35e639b33d7c58a0d89e47df7fe11838584d" ' 0(2974) parse_headers: flags=-1 0(2974) check_via_address(208.xxx.xxx.xx, 208.xxx.xxx.xx, 0) 0(2974) DEBUG:destroy_avp_list: destroying list (nil) 0(2974) receive_msg: cleaning up 0(2974) SIP Request: 0(2974) method: <ACK> 0(2974) uri: <sip:8889997777@206.xx.xx.x> 0(2974) version: <SIP/2.0> 0(2974) parse_headers: flags=1 0(2974) Found param type 232, <branch> = <z9hG4bK40f0830c>; state=6 0(2974) Found param type 235, <rport> = <n/a>; state=17 0(2974) end of header reached, state=5 0(2974) parse_headers: Via found, flags=1 0(2974) parse_headers: this is the first via 0(2974) After parse_msg... 0(2974) preparing to run routing scripts... 0(2974) parse_headers: flags=4 0(2974) DEBUG: add_param: tag=2d38da4c38ea547b795405b32554c3bb.f3cb 0(2974) end of header reached, state=29 0(2974) DEBUG: get_hdr_field: <To> [73]; uri=[sip:8889997777@206.xx.xx.x] 0(2974) DEBUG: to body [<sip:8889997777@206.xx.xx.x>] 0(2974) DEBUG: sl_filter_ACK : local ACK found -> dropping it! 0(2974) DEBUG:destroy_avp_list: destroying list (nil) 0(2974) receive_msg: cleaning up