How would the domain be developed, what type of uses would you think the name would have ?
These are important things to consider when estimating a domain's value.
At the moment the search results for those keywords point almost exclusively to at&t. So a brand would have to be built on this term which is totally different to their services, if the domain is to have any value.
Monetization via parking is not a good idea because the traffic would belong to at&t, which would probably result in at&t sueing for cybersquatting.
Hope this helps.