Let's say you have a telephone hang-up.

You don't even answer the phone much of the time because you think it invades your privacy. And you don't own an answering machine - not so much because you think it's crummy to screen calls but because if someone leaves a message, you feel obligated to call back.Of course you'd never have a beeper or a pager or a car phone or an answering service - you don't even have "call waiting."

Now comes a new phone feature that has the capacity to, well, really get on your nerves: It's "MCI Messenger," a service of long-distance carrier MCI Communications Corp. that promises to "send a message you record to anyone, any time."

Phone curmudgeon that you are, you immediately see this means other people can try to send recorded messages to you - all the time.

Here's how it works: If you're gone (or in your case, just not answering the phone) or if your line is busy (more likely in your case, off the hook) the caller can redial the MCI Messenger number or, if making an MCI card call, just punch in a code.

The caller follows instructions to record his or her message - up to one minute, 10 seconds long - and MCI Messenger takes it from there.

In 15 minutes, it calls you back. If there's no answer, it automatically calls you every 20 minutes for the first hour and continues calling back hourly for up to five more hours. The caller can even delay message delivery for up to 48 hours.

If you should answer one of these calls, you would hear a short recorded introduction and then, in the caller's own voice, the message. The service repeats the message twice and, if the caller has left one, gives the number where you can call back.

This costs the caller $1.60.

MCI Messenger is just now being introduced in the Northeastern and mid-Atlantic states and is to spread to the rest of the country this spring. To promote the service, MCI quietly inserted an announcement into phone bills recently, offering the first message free if the caller tries it within 30 days.

Consider yourself forewarned.

Is this just another device to make sure someone gets billed for every phone call attempted?

Marion Jordan, MCI's senior manager of corporate marketing, says it's much more - a lifesaver for harried pay-phone users. "We think this service is most valuable to people who are traveling because they have the most need to get a message through."

MCI's huge competitor, AT&T Co., offers a similar service, but as Jordan points out, the only way callers can access it is to dial a separate 800 number.

However, as AT&T spokeswoman Nancy Smith rather coolly notes, "We introduced it a year earlier" - last January. But who's heard of it?

AT&T calls its messaging service "VoiceMark." It basically works the same as Messenger but has more options.

Available not only anywhere in the United States - but also in 150 foreign countries - VoiceMark's one-minute messages can be scheduled for delivery right away or up to a week hence. Delivered automatically, they cost $1.75. Person-to-person deliveries, introduced by a real live person, cost $2.50. Another option allows the message recipient to re-cord for free a one-minute reply, which the original caller can retrieve free by calling VoiceMark.

Another carrier, US Sprint, does not yet offer the service.

To some, such a service probably could prove handy to prevent divorces ("Hi, honey, I'm going to be a few days late coming home from work") and other inconveniences.

For others who may see the service as a threat, remember: It still can't get to you unless you pick up the phone.

Of course, somebody somewhere is probably working on that.