An approach provides interdomain traversal to support packetized voice transmissions. A signaling message is received for establishing a voice call from a first endpoint associated with a first domain to a second endpoint associated with a second domain. The first endpoint queries a STUN (Simple Traversal of UDP (User Datagram Protocol)) server to determine information relating to a firewall and network address translator that the first endpoint is behind, and to log into a TURN (Traversal Using Relay NAT (Network Address Translation)) server configured to establish a media path between the first endpoint and the second endpoint. A first proxy server serving the first endpoint communicates with an ENUM (Electronic Number) server to convert a directory number corresponding to the second endpoint to a network address. The first proxy server communicates with a second proxy server serving the second endpoint to establish the voice call. The STUN server, the TURN server and the ENUM server are maintained by service provider. The first endpoint is authenticated to permit exchange of a media stream over the media path. The media stream is relayed, if the first endpoint is successfully authenticated.