Server Side I need to identify the carrier (ATT, Verizon, etc.).
On iPhone you can do it with the firmware number. I was thinking that Android's device + build information in the UA might do the trick.
Has anyone done this. I do not have client side access so Telephony Manager is not an option.
Unfortunately, User Agent won't do the trick.
You can get the carrier/network information for any device by using a geolocation service.
For at least one service that provides that level of detail, look at the answer to this similar question: How to determine if incoming IP address is from a mobile carrier?
You can probably find more.