Yes, the Linksys ATA's have no dial tone when it's not registered (I'm using a Linksys SPA2102). I don't know that much about the HT502, but I did a little research and you should be getting an IVR message when you pick up the handset that says "Device not registered". I took a quick look at the HT502 user manual and it states you will only get this message if the "Outgoing Call Without Registration" is set to NO. I suspect your ATA has been mistakenly programmed with this option set to YES. Just contact support and have them change this setting in your ATA. The information below is from their User Manual:

“Device not registered” - This prompt will be played immediately after off hook If the device is not register and the option “Outgoing Call without Registration” is in NO

"Outgoing Call without Registration" - Default is No. If set to “Yes,” user can place outgoing calls even when not registered but is unable to receive incoming calls.