Spa-3102 or similarrouting after failing to go via Asterisk

SPA-3102 will it do the job I require ?, basically I wish the device to first try routing calls via our server but if Asterisk cannot route the call then I wish the call to go via the PSTN (FXO Port)
Can this be easily achieved with the SPA-3102/3000 ?
Currently I use Asterisk and the CPE I use is a Dlink 1102S which does the job but is just too large and not in volume supply.
An idea toyed with so far was to create an account for the FXO port, but this would not be required and would double the accounts required as the only person allowed to access the FXO should only b the end user.
Thoughts on this much appreciated.