If i place a call to one of my pbx land lines to one of my voip numbers, the call should be routed by Askozia and should not really go on the land line....
Perhaps you meant
from the first one to the second?
Is this as designed or should it be considered a bug?
I think the answer is no for both.

The way the pbx route the calls depends on the patterns you put on the provider configuration, at this time out of the box nothing can reach the outside world.
Sure this exotic feature can be easily implemented but... is really useful? I'd used for long time a pbx offering that feature (the only one doing that according to my knoledge) and never appreciated such kind of fake routing.
However if you like it open the integrator panel and append some lines to extensions.conf:
[internal](+)
include => fakeroutemynumbers
[fakeroutemynumbers]
exten => 012345678,1,NoOp(Reroute own number)
exten => 012345678,n,Set(NOVOICEMAILWHENBUSY="yes")
exten => 012345678,n,Gosub(macro-main,s,1(SIP/100,100,100,tTo,))