Hi there
If you need to keep your services up and running all time, then you have to plan for a complete end to end redundancy (both hardware and applications wise). Here in your case, the PSTN breakout (the 4331 voice gateway) and the FW is a single point of failure and you dont have nothing to rely on. In a Single FW topology that serves your both internal and external users, it acts as a shared resource for these two zones. The main issue that can come up here is a denial of service attack (DOS) on the firewall from WAN can disrupt services on the LAN when you run out computing power on those FWs. On the other side, Of course, having two firewalls increases your administrative overhead, like you have to maintain two firewalls, backing up, patching to recommended. etc etc.
When you have 2 voice gateways, you can split the PRIs across two gateways so that if one goes down, the other takes care of the call routing (External calls) without any manual intervention.
Again, it all depends on how you want your network designed and also depends on your budget, requirements, business needs etc
Hope this helps!
Cheers
Rath!
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