320x480 div, how to make it scale on all phones?

320x480 div, how to make it scale on all phones?

At the moment I'm using the following viewport code with a 320x480
portrait phone ad:
<meta name="viewport" content="width=device-width; initial-scale=1.0;
maximum-scale=1.0; user-scalable=0;" />
For me, it scales well on an iPhone3, iPhone3GS, iPhone4, iPhone4S,
iPhone5, Samsung galaxy S3, a Sony Erricson phone and so on
The agency who is serving these ads need it to work on all phones and tell
us that it's not scaling, but wont offer which devices it wont scale on :/
Is there anything else I can do in regards to that meta viewport tag to
ensure it scales to the width of a device?
Thanks