Yes, It's 1720 TCP, not UDP. Portusage docs don't mention 1717-1719, only 1720 if you go external. Not sure about BLF, don't see those ports in the doc either, but I've not been asked for that feature. I'll have to test that stuff sometime.
A vpn between sites makes things way better, more secure and no ports to open. You still need to build tables though. I've put in a few network systems with and without licensing. For just intercom calls it works fine without licensing. If you're transferring outside calls between systems a license means less problems, full tech support and a happier customer/boss. I'd set it up without networking, try some calls and then activate demo mode(60 days) which turns on full networking amongst other things, and try that out. You have 6 shots at demo mode on emg80's or ucp, so use them sparingly. Make sure to turn networking mode on in system network settings and turn off the attendant alarms for demo mode and networking expiry to make your life easier.
You can also do features like paging across the network, and remote dial tone from the other system, but get networking working first then try the tricky stuff.
The number of total active voip channels between sites, remote phones and sip lines will affect service. Default on an emg80 is 2 channels, so only 2 calls at a time. It needs at least 4-6 channels if you're networking, have a couple sip lines and a couple of remote phones, or people will get dead air and not be happy.
The built in networking works well, use sip for in and outgoing calls and that's it.
Apparently you can also network these to other systems. Never tried it but you never know...