I am having the same problem with a new Polycom VOIP phone. Verizon replaced my 4+ year old FiOS router and ONT, but I only get one way audio. Especially odd is that it's not always outbound voice. my Verizon wireless, the voice is audible inbound. However, it never changes direction on any line on subsequent attempts. If the voice is heard outbound (which is most), that's always what happens.
FiOS itself doesn't block any ports except for outbound port 25 which is a popular port for spammers, but you have a router over there. A Router and a firewall's job is to block ports. So you'll need to review your manual and see what ports need to be forwarded and then try to apply the port forwarding rules in the router. That is Phone Specific so it differs from one voip provider to another
Don't forget, After you configure your Port forwarding we suggest that you might want to test your phone connection after completing a power cycle of the ATA. This will allow for a registration process with the ATA reaching out and downloading a configuration file from your provider, which should result with dail tone to the ATA. Then make and outbound call, as well as make a call inbound to your connection.