Hi, when FS calls to a natted user, it generates an INVITE with a Request URI as follows: INVITE sip:user@XX.XX.XX.XX:XXXX;transport=udp;received="XX.XX.XX.XX:XXXX" SIP/2.0 Well, this is not valid since a SIP URI parameter cannot contain quote symbol ("). You can verify it in RFC 3261 - page 222 (look for "other-param"): ------------ uri-parameters = *( ";" uri-parameter) uri-parameter = transport-param / user-param / method-param / ttl-param / maddr-param / lr-param / other-param transport-param = "transport=" ( "udp" / "tcp" / "sctp" / "tls" / other-transport) other-transport = token user-param = "user=" ( "phone" / "ip" / other-user) other-user = token method-param = "method=" Method ttl-param = "ttl=" ttl maddr-param = "maddr=" host lr-param = "lr" other-param = pname [ "=" pvalue ] pname = 1*paramchar pvalue = 1*paramchar paramchar = param-unreserved / unreserved / escaped param-unreserved = "[" / "]" / "/" / ":" / "&" / "+" / "$" ------------- This causes Twinkle sofphone droping the incoming request since it's malformed. Quote symbol (") should not be used in a SIP URI parameter in order to be SIP compliant. Since I just use Twinkle (the best and more SIP compliant softphone for Linux) I can't test FreeSwitch for now :( Best regards. -- Iñaki Baz Castillo