Update: Recovery attempt via telecom operators failed - the number is stuck in limbo between two carriers. While this didn't work in my case, other European users may still recover their number by either:
1. Starting a new 2-year contract
2. Paying a one-time fee for a preferred number (~150 EUR in my case)
Tip: Contact your local carrier to check if number recovery is possible, or if you can request a specific number for a new contract.
Currently pursuing Plan B: Submitted a Data Access Request [1] to Google in hopes of getting my data back.