PHP SoapFault not caught by exception handlers

I am new to PHP exception handling and SOAP. For some reason I cannot catch a SoapFault. I don't know why. The soap server is not mine.

try { 
    $contact_id = $objSoapClient->getContactIdFromVisitorId('12345');
} 
catch (SoapFault $sf) { 
    echo "Soapfault"; 
} 
catch (Exception $e) { 
    echo "Exception"; 
}

I am purposely passing in the bad id 12345. When I enable errors I see the following message SoapFault exception: [SOAP-ENV:Client] Invalid Visitor ID. However, my catch SoapFault block nor my catch Exception block ever get hit. Why?