If you and your contacts have a stable IPv6 (or even public IPv4), you
don't need a sip service. One way to do that is with an IPv6 VPN
like Cjdns or Yggdrasil. Use linphone in peer to peer mode.
If you want a bridge to the century old POTS network, linphone doesn't
seem to offer that. I use https://diamondcard.us - there are hundreds
If you want to support linphone, they sell supported licensed products
geared toward business.
P.S. I just thought to wonder if you can call sip.linphone.org accounts
from linphone in peer to peer mode and vice versa. I'll have to try it,
but probably not for IPv6 as sip.linphone.org has no IPv6 in DNS.
A long time ago, I did try port forwarding 5060 with nf-conntrack-sip to
a fixed private IPv4 running ekiga, and was able to talk to users on sip
Stuart D. Gathman <[hidden email]>
"Confutatis maledictis, flamis acribus addictis" - background song for
a Microsoft sponsored "Where do you want to go from here?" commercial.