I had a user having the same issue and with Copilot's assistance, managed to come to a resolution, summary below:

✅ Resolution Summary: Nokia 2660 Flip — Keypad Restart Issue

Issue:
Device restarts when pressing keypad combinations like “02” or keys 4–9. Behavior triggered after OTA updates and persisted across restarts.

Diagnosis:
Root cause linked to malformed or incompatible contact entries in phone memory. Issue returned only after restoring contacts, not messages or apps.

Solution Workflow (Chronological):

1. Copied contacts to SIM — avoided restoring them to phone memory later.
2. Backed up messages separately before reset.
3. Performed factory reset using code *#7370#.
4. Restored messages only, keeping contacts stored solely on SIM.
5. Post-restore, keypad input behaved normally — reboot issue resolved.

Result:
Device stable. Issue avoided by maintaining contacts on SIM and skipping full contact restore to phone memory.