First, apologies are due for not continuing the original thread; I was interacting with the mailing list via gmane.org and the recent messages, including mine, don't appear to have propagated to the newsgroup. I solved my difficulty with making outbound calls to my ISP's SIP service. The problem turned out to be the MTU setting of my ppp0 interface: reducing it to 1496 bytes allowed the FreeSWITCH invite packet (with the md5 authentication) to be fragmented and received by the remote side.