Registering AT&T Dect 6.0 base station with Panasonic Dect 6.0 handset?

No. In order to support interoperability among a mix of vendors, the handset must support GAP (Generic Access Profile): This is widely supported in Europe, but not in the US. However, even if GAP were supported, there is not test or certification requirement for cross vendor interoperability. The GAP spec also does not cover optional features, such as answering machines. If the base has an answering machine, it's unlikely that another vendors handset can program the answering machine as the menus are in the handset, not the base.

OK thanks for that.I just thought that as both SEEM to use the same freqs & (bare) specs and both support interop with additional handsets, they might cross-support.

This bit of the referenced page certainly is confusing: "Most consumer- level DECT phones and base stations support the GAP profile, even those that do not publicize

Frequencies: 1880 - 1900 MHz in Europe 1900 - 1920 MHz in China 1893 - 1906 MHz in Japan 1910 - 1930 MHz in Latin America 1920 - 1930 MHz in US and Canada

DECT 6.0 products do not support "GAP", therefore phones of differing brands are not guaranteed to work together. E.g. A GE 28105EE1 DECT 6.0 headset will not receive calls if registered to a Uniden base unit.

Actually, that's not quite true. GAP is not required for DECT 6.0 (which is not a standard but rather a marketing term). However some vendors advertise GAP compatibility. For example: Whether it actually works between different vendors is doubtful.