Well, yes in theory you could have 255 VoIP connections, all of the phones
could be analog ones connected via ATA's. You could also connect similar
number of WiFi VoIP phones.
In practice you won't be able to set up port forwarding and registrars for
all of the phones to use them as separate lines. There are VoIP PBX systems
for that.

I am not familiar with the 716g, but on other routers that have QoS facility
you can set separate priorities for different kind of packets, not just
voice. This will work for all the ports.
Regards,
Martin