This is what I would think would be ideal:
-Get a VoIP account with a third party (FWD, MySipSwitch, Gizmo OR anything else you can think of that's simple/open and reliable to deliver SIP)
-Register this in your VoXalot acct setting SIP register to Yes
-Use the forwarding rules to ensure all calls coming to that provider are sent to VM automatically
-Now have your SPA send PSTN callers to the third party destination after 20sec, which in turn will land them in your VoXalot VM right away
Because you are now using Call Forward rules to send PSTN callers to VM, you can raise your VM answer time for regular calls coming over VoXalot or any other of the registered providers
Now, whether that helps or not, remains to be tested