An approach provides interdomain traversal to support packetized voice transmissions. A request for establishing a voice call is received from a source endpoint behind a first network address translator of a first domain, wherein the request specifies a directory number of a destination endpoint within a second domain. A network address is determined for communicating with the destination endpoint based on the directory number. Additionally, existence of a second network address translator within the second domain is determined. If the network address can be determined, a media path is established between the source endpoint and the destination endpoint based on the network address to support the voice call.