The calculator uses a deterministic (non-random) algorithm that combines the IMEI and the network lock code to generate a unique unlock code . This is why two phones with the same IMEI (impossible) would have the same code, but two phones with different IMEIs will have different codes.
Using a DCT4 calculator to unlock a phone you own is generally considered in most countries today, but it was legally contested in the past. Unlocking a subsidized phone before completing a contract could violate the terms of service. However, the primary ethical defense remains: You should own the hardware you paid for. Nokia Dct4 Calculator Online
While many original sites have gone offline, you can still find functional calculators or downloadable software: UnlockPedia Nokia Dct4 Calculator Online