Normally there's only one ground station and multiple aircraft on one frequency, so it makes sense that there are different rules.
Here's an example
"XXX Radio, G-ABCD request departure information"
"G-CD, XXX Radio, runway YY, surface wind zzz/zz, QNH 1013" <- A/C callsign abbreviated by AFIS.
"Runway YY, QNH 1013, G-CD" <- AFIS callsign completely omitted by A/C
This is within a single conversation. But for instance during a transit of a CTR you might have multiple of these conversations. The first one will start with the full callsign, e.g. "XXX Tower, G-ABCD request transit". But any next conversation might be initiated with just "XXX" or "Tower" or even leave out the callsign of the ground station altogether, for instance in a position report:
"G-CD is Bravo"
"G-CD report Tango"
"Wilco Tango, G-CD"