Just wondering, why is that? It seems like a lot of effort instead of just sending a BYE when the phone is hanged up. I don’t see any legitimate reason for this “feature” to exist - if anything it makes scams & eavesdropping by a malicious remote caller easier.

Per one of the articles I found on it, British Telecom apparently set the delay to 3min. The rational (if article is to be trusted) is that customers wanted the ability to hang up one phone and pick up another phone in the same home without disconnecting the call.