Codes sent by text message

Some annoying websites insist on authentication by requiring me to enter the number they send by text message.  For some reason my google voice number is never identified as a real phone number and I have to use my old phone with the $10/year SIM to receive their code.
WTF?  Why is the google voice number not a REAL phone number?
--
Cheers, Bev
   "Mr Panetta also revealed that the US Navy Seals made the final
    decision to kill bin Laden rather than the president."
                                --S. Swinford, The Telegraph
   [Aside from that minor error, those Seals did a fantastic job!] --Bev