All I remember from seeing this as a kid is that, in the final scene, River Phoenix asks the hot government agent for her phone number. The number she gives is a real non-555 number--pretty sure it was 818 area code?. You could call that number and hear a message from the character for many years.
You're gonna love the answer to that. Disney's 1994 family holiday treat, The Santa Clause, starred Tim Allen as a guy who (I'm remembering this as I'm typing it and holy cow) sort of kills Santa Claus by startling him into falling off the roof. He then puts on the Santa suit and becomes the archetype himself. But in that movie, there's an off-hand reference to a phone number: 1-800-SPANK-ME, meant entirely sarcastically. Turns out, either that was already a real number or some enterprising pornographer recognized that there's no such thing as bad publicity, and precocious youngsters who called the line after watching the movie were invited to pay $5/minute for "the hottest phone line in America".
Tossing a few ARG links here, since the heyday of larger games (and especially ones with PTSN connectivity) is old enough some HNs might not have experienced them.
I really enjoyed the original Valve's Portal BBS you could dial into Aperture Science and read on the history. The writers at Valve really wrote some great silly backstory about a shower curtain company developing quantum portals, as some sort of tool to get in and out of showers. I'm not sure that's cannon anymore though.
I think that still is (per Portal 2), but some of the original details aren't – like how Cave Johnson poisons himself, and how much his excellent decision-making was influenced by said poisoning.
Was this the only movie ever to do this?