Source connect but pricey if you’re only going to use it once a week or so.
They have a free 320K instant messaging AV package that knocks the socks off skype that’s also handy for live collaboration in pro tools sessions. It all depends on the depth of integration you want. Top studios the world over use it to connect session and clock and sync and lock.
I have a podcast of the owner of IPDTL a UK company btw, who saw a gap in the market in the low to mid end pro audio market using local servers, but there’s a subscription model in place and the traffic loads can vary…
If you just need basic talk to talk then ISDN still works well for live direction between director/producer and vocal artist. You can pick encoders up for half nothing on Ebay but you are renting 2 analogue copper phone lines.