Is there anyway to connect SFP cage to banana pi R2?
answer : we not support SFP on BPI-R2 , it need to design a new hardware to support SFP. BPI-R2 just have POE support (option)
What about availability of Asterisk PBX hard with FXS/FXO/T1/E1 cards? Cost?
yes, sinovoip have support many type asterisk card for VOIP,you can see here
sinovop also design ippbx
so ,you just need to design a ippbx case to do your IPPBX product .