No inbound audio after a few seconds but has outbound audio is absolutely a remote (phone side) firewall/router issue... Something changed, a firmware update or settings change, a different port is being used and triggering IDS/IPS, or something might have gotten pushed by the ISP or auto update in the router.
I would first restart the router and test... sometimes this is enough to clear it up. but otherwise a quick Google search doesn't give much hope for this router, it is seems pretty poorly rated and a lot of the features are disabled or locked out by Virgin Media and my gut says just from what you are saying and reading a few online reviews, the issue is likely with the router.
If you can do a remote session and look for any firewall or security setting and try turning them off, but from just the quick Googling I did, it looks like this is one of the least configurable routers out there. Otherwise, I would consider an upgrade of the router to a different model, and if at all possible move them to a dumb bridge modem and get a standalone wireless router, even a simple $50 TP-Link router would probably be a world better than this thing.
