[chronomex]

Not quite true. They don't chain like that. A tells O that B has become a better choice, so a complex dance happens where:

a new circuit is established from O to B,

O tells A to tell the handset to move over to B-owned network infrastructure,

the handset roams over to B,

O drops the circuit to A.

This is a special case of Inter-MSC Handover, and you can read the spec at https://archive.org/stream/etsi_ts_123_009_v03.03.00/ts_1230...

The relevant section is Sec. 7.3.2, "Description of the subsequent handover procedure ii): MSC-B to MSC-B'".

(They call your "O" by "A", your "A" by "B", and your "C" by "B'" (B prime). And an MSC is a Mobile Switching Center that usually handles about half a city's worth of towers.)

[spc476]

Is that while a call is in progress? That sounds like what happens when a cell phone that is not in use, transfers from switch to switch like you describe.

Or the instructor I had taught us outdated information.

[chronomex]

Handover is for established calls. I'm not surprised your instructor wasn't quite spot-on, GSM is not a simple protocol.