OK, update: Resolved our Sangoma Talk problems with the assistance of support (thanks Chris!).
The IP addresses that ST uses, found by doing a DNS lookup on all.sipis.acrobitz.cz, had changed from what we had in the firewall, both FreePBX and corporate. Added the new ones, removed the old ones but still having problems. Realized today that we had other corporate firewall entries blocking some of those addresses, which weren’t used by ST at the time we added the blocks. Removed those blocks and it’s working again.
It would be helpful to know when those IP addresses change, any chance that a notification can be set up to do that? It’s not like you’re changing them overnight, there has to be planning of changes. SAP Ariba has been emailing me for months that their certificate or IP or whatever is changing.
{Edit} This is the resolution for this post, too: Sangoma Talk mobile app