this kind of phone is quite good and has more function.
 Support SIP,IAX2,H323 protocol
    NAT, Firewall.
    It can register two SIP account and one IAX2 account in the same time.
   

All there 3 account can be used as  called anytime,and for the caller,it can be slelected by setting different dial prefix.
 if you want to know more ,please let me know.