As I type this, the Voyager & Ufone VoIP outage is not yet fully resolved. We've had a quick chat internally about mitigating this sort of thing....
Where there is a DDoS attack (or another attack that overwhelms a VoIP system), presumably if you had a rule that forwarded calls to mobile phone if the on-prem VoIP PBX was unavailable, this forward would not actually work because the provider's VoIP systems were not in any sort of state to actually do the forward to mobile?
Likewise having a trunk with VoIP Provider A and a second trunk with VoIP Provider B..... if Provider A suffers a DDoS attack or similar, calls won't be forwarded to Provider B because Provider A's systems are not in a state to initiate that forward.
Perhaps it wouldn't be a terrible idea to have a hybrid solution with the client's main number being an analog/ISDN line with a converter into an on premises VoIP PABX, but all overflow lines and outgoing calling lines are VoIP. So if this kinda thing happens again, at least the client can still receive calls one at a time, and presumably request the analog/ISDN provider change the diversion from a forward-to-VoIP-line to a forward-to-mobile.
Thoughts?