Implement an in-memory cache that will keep phone numbers that are already registered in OBS e8

[Koufan-De-King]

Description:
Develop an in-memory cache in the OBS to store already registered phone numbers, enabling efficient lookup and reducing database queries.

Requirements:

• Implement a thread-safe in-memory cache.
• Store registered phone numbers with efficient lookup.
• Ensure cache eviction or persistence if needed.

Acceptance Criteria:

• Registered phone numbers cached successfully.
• Cache lookup time optimized.
• Passed unit and integration tests.
• Code reviewed and approved by the team.

Estimate: 8