Does any one have idea of how voluntary pention contribution works? i.e.

1) which office to be contacted if one wants to make volutnary contribution?

2) Assuming contributions are paid to BFA. Does Auslanderbehoerde accept these contributions or the contribution made only through insurance company (AOK, etc...) are considered.

3) Any idea, how is the amount of contribution calculated?

I think no one needs it.
look here:
http://www.trust7.com/de/forums/integration/60_rentenbeitrags_are_not_necessary#msg7365

ArbeitsBerechtigung - PWP or
AusländerBehörde - Foreign Office

ArbeitsErlaubnis - WP or
AufenthaltsErlaubnis RP

AufenthaltsBeWilligung - Student visa

NiederlassungsErlaubnis - PR

AB, AE, ABW NE.... etc mean ?