The reason why I asked about the Avaya system is because the IP phones are typically configured to operate off of DHCP. So if you're having two DHCP servers running on the same layer 2 segment, you're going to have problems. This could explain the weird behaviors you're seeing.
As far as securing your network, it's all about how paranoid you want to be. Many times, a standard SPI (stateful packet inspection) firewall is enough. All current firewalls are SPI firewalls instead of the old school packet filtering type. If you want to step up your firewall capabilities, you'll have to look into either an integrated UTM (unified threat management) firewall appliance or a next generation firewall which has DPI (deep packet inspection).
As far as securing your network, it's all about how paranoid you want to be. Many times, a standard SPI (stateful packet inspection) firewall is enough. All current firewalls are SPI firewalls instead of the old school packet filtering type. If you want to step up your firewall capabilities, you'll have to look into either an integrated UTM (unified threat management) firewall appliance or a next generation firewall which has DPI (deep packet inspection).