What is the overall protocol-level sequence of a 2G phone from power-on until it can use mobile data?

Network graph awareness → network selection (RF scan, sync, BCCH decode) → cell selection → LOCATION UPDATE (registration) → GPRS LOCATION UPDATE / attach (data registration) → PDP CONTEXT (tunnel to the internet).

* The full 2G chain from power-on to the internet. *

The complete chain, in order:

1. The classic GSM (2G) network is the playing field: MS ↔ BTS ↔ BSC ↔ MSC, with HLR/VLR/AUC behind it
2. Network Selection: RF power scan → frequency + sync burst detection → BCCH decode (learn MCC/MNC/Cell-ID)
3. Cell selection state machine — the phone settles ("camps") on the best suitable cell
4. LOCATION UPDATE — registration with the network (IMSI Attach at power-on)
5. GSM Control Plane protocol stack carries all of this signaling
6. GPRS LOCATION UPDATE / GPRS Attach — separate registration for the data domain (toward the SGSN)
7. PDP CONTEXT activation — the tunnel to the GGSN; the phone gets an IP address and reaches the internet
8. The same concepts carry over to UMTS with new names on the radio side

Why learn the 2G version? Because it's the simplest complete instance of the pattern — and every later generation (UMTS, LTE, 5G) is a refinement of exactly this sequence: find network → authenticate & register → establish session. Master it once in 2G and the LTE attach procedure reads like a familiar story with renamed characters.

Go deeper:

• Harald Welte — What happens on a protocol level when I switch on my phone? (CCC) — a step-by-step live walkthrough of exactly this power-on → register → data sequence in real protocol messages.
• Network switching subsystem (Wikipedia) — reference for the MSC / HLR / VLR / AUC core elements behind the MS↔BTS↔BSC↔MSC playing field.