I have been testing the failover provider feature in ASKOZIA 2.0.1 and it works fine, but you have to be aware of knowing how and when does ASKOZIA 2.x jump to a failover provider.
As soon as you configure 2 providers and you can see that ASKOZIA generates the following code for the "outgoing provider" of your main provider (the first outgoing option, not the failover option) in the /etc/asterisk/extensions.conf configuration file:
; outgoing provider : sylcomgsm2015
[SIP-PROVIDER-17914080404a89b80e4508c]
exten => _2.,1,NoOp(outgoing call to provider: sylcomgsm2015)
exten => _2.,n,Set(ORIGEXTENSION=${EXTEN})
exten => _2.,n,NoOp(original extension = ${ORIGEXTENSION})
exten => _2.,n,Dial(SIP/${EXTEN}@SIP-PROVIDER-17914080404a89b80e4508c,,To)
exten => _2.,n,NoOp(DIALSTATUS = ${DIALSTATUS})
exten => _2.,n,Goto(SIP-PROVIDER-17914080404a89b80e4508c-${DIALSTATUS},1)
exten => SIP-PROVIDER-17914080404a89b80e4508c-NOANSWER,1,Hangup()
exten => SIP-PROVIDER-17914080404a89b80e4508c-CANCEL,1,Hangup()
exten => SIP-PROVIDER-17914080404a89b80e4508c-BUSY,1,Hangup()
exten => SIP-PROVIDER-17914080404a89b80e4508c-CONGESTION,1,NoOp(Failover needed, now using : Gateway RDSI)
exten => SIP-PROVIDER-17914080404a89b80e4508c-CONGESTION,n,Goto(IAX-PROVIDER-12629461584b203ac38302e,${ORIGEXTENSION},1)
exten => SIP-PROVIDER-17914080404a89b80e4508c-CHANUNAVAIL,1,NoOp(Failover needed, now using : Gateway RDSI)
exten => SIP-PROVIDER-17914080404a89b80e4508c-CHANUNAVAIL,n,Goto(IAX-PROVIDER-12629461584b203ac38302e,${ORIGEXTENSION},1)
And as you can see ASKOZIA 2.x recognizes the following SIP codes that it gets in the DIALSTATUS variable: NOANSWER, CANCEL, BUSY, CONGESTION or CHANUNAVAIL. If your provider generates any other SIP code, ASKOZIA will not process it and failover will not happen. Also, as you can see, ASKOZIA 2.x only does failover on CONGESTION and CHANUNAVAIL SIP codes, so even if your provider generates a BUSY SIP code, which is very common, ASKOZIA 2.x will not jump to a failover provider. As you can see in the code, this works this way by design. If you want to know why, you would have to ask Michael - the designer of the system.
In fact, I was a little surprised with this code, especially with the BUSY SIP code behaviour. I expected ASKOZIA 2.x to jump to the failover provider on "BUSY", but it does not. You can use any provider as a failover provider but on your main provider you have to make sure that when you have a call or several calls, or the link is down so you cannot make any more calls using the main provider, this provider has to return a SIP code telling ASKOZIA that it is either CONGESTED or that the CHANNEL IS UNAVAILABLE for failover to work properly.