Use your BT free evening/weekend call allowance.

Create a new context in extensions_custom.conf

[custom-timeselect]
exten => _[*0-9]!,1,GotoIfTime(05:30-17:59|mon-fri|*|*?dayselect)
exten => _[*0-9]!,n,Dial(SIP/bt/${EXTEN})
exten => _[*0-9]!,n,Dial(SIP/voipstunt/${EXTEN})
exten => _[*0-9]!,n,Goto(app-blackhole,congestion,1)
exten => _[*0-9]!,n(dayselect),Dial(SIP/voipstunt/${EXTEN})
exten => _[*0-9]!,n,Goto(app-blackhole,congestion,1)
exten => h,1,Hangup()

This assumes that you have named a trunk bt (for evening and weekend free calls) and you have a trunk called voipstunt for any other calls.

Create a CUSTOM trunk. In the Dial text box, enter this:

Local/$OUTNUM$@custom-timeselect

Set up your Outbound Route to use the custom trunk rather than calling either of the two trunks directly.