FXO - 2 or 4 or 6 or 8 Port Gateway in either H.323 or SIP
 
   
     
 
FXS - 2 or 4 or 6 or 8 Port Extension in either H.323 or SIP
 
 
     
 
FXO+FXS - 1 or 2 or 4 Port Gateway in either H.323 or SIP