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

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_123009v030300p#page/n33/mode/2up 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.)