Hello Henning, There is not much going on. The initial handshake is not even there. ssldump port 5061  -k /etc/letsencrypt/live/openfabrik.com/privkey.pem New TCP connection #3: dev-sbc.openfabrik.com(35795) <-> 52.114.132.46(5061)3    0.0244 (0.0244)  C>S  TCP RSTNew TCP connection #2: dev-sbc.openfabrik.com(38665) <-> 52.114.76.76(5061)2    0.0788 (0.0788)  C>S  TCP RSTNew TCP connection #1: dev-sbc.openfabrik.com(40740) <-> 52.114.7.24(5061)1    0.1967 (0.1967)  C>S  TCP RST On Sunday, May 9, 2021, 03:22:48 a.m. EDT, Henning Westerholt &lt;hw(a)skalatan.de&gt; wrote: #yiv5610585594 #yiv5610585594 -- _filtered {} _filtered {} _filtered {}#yiv5610585594 #yiv5610585594 p.yiv5610585594MsoNormal, #yiv5610585594 li.yiv5610585594MsoNormal, #yiv5610585594 div.yiv5610585594MsoNormal {margin:0cm;font-size:11.0pt;font-family:sans-serif;}#yiv5610585594 a:link, #yiv5610585594 span.yiv5610585594MsoHyperlink {color:#0563C1;text-decoration:underline;}#yiv5610585594 span.yiv5610585594E-MailFormatvorlage19 {font-family:sans-serif;color:windowtext;}#yiv5610585594 .yiv5610585594MsoChpDefault {font-size:10.0pt;} _filtered {}#yiv5610585594 div.yiv5610585594WordSection1 {}#yiv5610585594 Hello Victor,   just a generic comment about the Kamailio version – if you start to create a new setup, you really should use the latest stable version of the 5.4.x branch. 5.2.x is already end of life.   About your specific issue, already looked into it with e.g. a network tracing tool?   Cheers,   Henning   -- Henning Westerholt –https://skalatan.de/blog/ Kamailio services –https://gilawa.com   From: sr-users &lt;sr-users-bounces(a)lists.kamailio.org&gt;On Behalf Of Victor Velo Sent: Sunday, May 9, 2021 4:48 AM To: sr-users(a)lists.kamailio.org Subject: [SR-Users] MS Teams SBC   Hello,   I'm trying to test a Henning's tutorial for Teams SBC described here:https://skalatan.de/en/blog/kamailio-sbc-teams.   A minor change at the MS Teams setup is just about the login. Now it requires MFA. The license I used is : Microsoft 365 developer subscriptions (E5). The domanin dv-sbc.openabrik.com is added + 1 user   PS> Connect-MicrosoftTeams (MFA) PS> New-CsOnlinePSTNGateway -Fqdn dev-sbc.openfabrik.com -SipSignalingPort 5061 -MaxConcurrentSessions 10 -ForwardCallHistory $true -Enabled $true   Everything seems to be OK at MS Teams side.   I'm using the Kamailio coming with Kazoo. Version 5.2.5 [root@dev-sbc kamailio]# kamailio -version version: kamailio 5.2.5 (x86_64/linux)  flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES ADAPTIVE_WAIT_LOOPS=1024, MAX_RECV_BUFFER_SIZE 262144 MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: unknown  compiled on 01:14:59 Nov 19 2019 with gcc 7.3.1       When Kamailio is probing MS Teams, the log shows the following errors (for the 3 GW):   hG4bKc188.6a606040000000000000000000000000.0#015#012To: <sip:sip2.pstnhub.microsoft.com;transport=tls>#015#012From: <sip:dev-sbc.openfabrik.com>;tag=d9a52ca041d9e6253dd2a7d5399056e3-2f56#015#012CSeq: 10 OPTIONS#015#012Call-ID:4ed99b8a73a96dcd-10234@155.138.140.233#015#012Max-Forwards: 70#015#012Content-Length: 0 May  9 01:26:17 dev-sbc kamailio[10234]: ERROR: <core> [core/tcp_main.c:618]: _wbufq_add(): (517 bytes): write queue full or timeout  (0, total 1100374016, last write 41683154 s ago) May  9 01:26:17 dev-sbc kamailio[10234]: ERROR: <core> [core/tcp_main.c:2673]: tcpconn_1st_send(): 52.114.76.76:5061: EAGAIN and write queue full or failed for 0x7fce434db860 May  9 01:26:17 dev-sbc kamailio[10234]: ERROR: tm [../../core/forward.h:293]: msg_send_buffer(): tcp_send failed May  9 01:26:17 dev-sbc kamailio[10234]: ERROR: tm [uac.c:648]: send_prepared_request_impl(): Attempt to send to precreated request failed May  9 01:26:17 dev-sbc kamailio[10234]: INFO: <script>: Sent out tm request to Teams: OPTIONSsip:sip.pstnhub.microsoft.com;transport=tls SIP/2.0#015#012Via: SIP/2.0/TLS 155.138.140.233:5061;branc   config:   tls.cfg   [server:default] method = TLSv1.2+ verify_certificate = yes require_certificate = yes private_key = /etc/letsencrypt/live/openfabrik.com/privkey.pem certificate = /etc/letsencrypt/live/openfabrik.com/fullchain.pem ca_list = /etc/letsencrypt/live/openfabrik.com/bc2025.pem server_name = dev-sbc.openfabrik.com   [client:default] method = TLSv1.2+ verify_certificate = yes require_certificate = yes private_key = /etc/letsencrypt/live/openfabrik.com/privkey.pem certificate = /etc/letsencrypt/live/openfabrik.com/fullchain.pem ca_list = /etc/letsencrypt/live/openfabrik.com/bc2025.pem   ====   event_route[tm:local-request] {     setflag(FLAG_LOCAL_REQUEST);       if(is_method("OPTIONS") && $ru =~ "pstnhub.microsoft.com") {            append_hf("Contact: <sip:dev-sbc.openfabrik.com:5061;transport=tls>\r\n");            xlog("L_INFO", "Sent out tm request to Teams: $mb\n");     }       xlog("L_DEBUG", "$ci|local|start $pr request $rm $ou\n");     xlog("L_DEBUG", "$ci|local|source $si:$sp -> $dd:$dp\n");     xlog("L_DEBUG", "$ci|local|from $fu\n");     xlog("L_DEBUG", "$ci|local|to $tu\n");       #!ifdef SIP_TRACE_ROLE     route(SIP_TRACE);     #!endif       #!ifdef PRESENCE_ROLE     route(PRESENCE_LOCAL_REQUEST);     #!endif   }   ==== # setid(integer) destination(sip uri) flags (integer, optional), priority(int,opt), attrs (str,optional) kamctl dispatcher add 2 'sip:sip.pstnhub.microsoft.com;transport=tls' 0 3 'socket=tls:155.138.140.233:5061;ping_from=sip:dev-sbc.openfabrik.com' kamctl dispatcher add 3 'sip:sip2.pstnhub.microsoft.com;transport=tls' 0 2 'socket=tls:155.138.140.233:5061;ping_from=sip:dev-sbc.openfabrik.com' kamctl dispatcher add 4 'sip:sip3.pstnhub.microsoft.com;transport=tls' 0 1 'socket=tls:155.138.140.233:5061;ping_from=sip:dev-sbc.openfabrik.com'   [root@dev-sbc kamailio]# kamctl dispatcher show dispatcher gateways sqlite_query: /usr/bin/sqlite3  /etc/kazoo/kamailio/db/kazoo.db select * FROM dispatcher ORDER BY setid;  1|1|sip:155.138.140.233:11000|9|0|zone=local;profile=sipinterface_1;duid=243;node=freeswitch@dev-sbc.openfabrik.com|added by nodes role 2|2|sip:sip.pstnhub.microsoft.com;transport=tls|0|3|socket=tls:155.138.140.233:5061;ping_from=sip:dev-sbc.openfabrik.com| 3|3|sip:sip2.pstnhub.microsoft.com;transport=tls|0|2|socket=tls:155.138.140.233:5061;ping_from=sip:dev-sbc.openfabrik.com| 4|4|sip:sip3.pstnhub.microsoft.com;transport=tls|0|1|socket=tls:155.138.140.233:5061;ping_from=sip:dev-sbc.openfabrik.com|       Is it a bug on the Kamailio 5.2.5 release or Do I miss something?   Thanks,   Victor