Here's some info on the different Linksys adapters:
The SPA-1001 has been discontinued and is replaced with the PAP2T.
The PAP2 has been discontinued and is replaced with the PAP2T.
The PAP2T is the latest model 2 FXS adapter from Linksys.
The SPA2102 is the latest model 2 FXS, router, adapter from Linksys.
The SPA3102 is the latest model 1 FXO, 1 FXS, router, adapter from Linksys.
-NA or -EU means that the device is not associated with a particular VoIP provider (unlocked). Devices without -NA or -EU usually have another -XX designation and are locked to a specific VoIP Provider.
If you are looking to purchase used equipment, make sure it is a -NA or -EU model. Other models are not truely "unlocked" and will not be supported by Linksys or other prodivers.
More information and data sheets for the PAP2T-NA, SPA2102, and SPA3102-NA are available on our store pages.