What are GG and DD phone numbers?

[Oldfart]

Lately I have been seeing ads listing phone numbers as GG or DD. Could someone please explain to this old luddite what those mean? I do have a mobile phone that can send and receive textual messages, by the way.

 

[jizzme]

bump, i would like to know as well...guessing day time and night time?

 

[MissingOne]

... I do have a mobile phone that can send and receive textual messages, by the way.

So do I. But I have no clue how to do it.