Many years ago, my elementary school teacher read to us a funny
answering machine message poem, and I am having trouble finding it
again now. I had thought it was by Shel Silverstein because it was
similar in tone and style to his children's poems, but my own searches
tell me it's probably by a different author after all. Here is what I
remember about the poem:

It was long (probably more than a dozen lines). It rhymed. It
enumerated various improbable possibilities for why the speaker could
not come to the phone (I think being in France or some other foreign
country might have been one of them). It ended with a request to leave
a message at the beep or something to that effect. I am fairly certain
that the poem was published (that is, that it appeared in print
somewhere, so it's not something my teacher just made up).

If anyone could find me the text of the poem, or just its title and
author, I would be extremely grateful.

I have found:

These words are lovely dark and deep 
But I've got promises to keep 
and miles to go before I sleep 
So leave a message at the beep. 
Now I lay me down to sleep; 
Leave a message at the beep. 
If I die before I wake, 
Remember to erase the tape. 

It and other answering machine messages (but not poems) are on
http://homepages.tesco.net/~scotsnet/o.f.carter/fun/fun007.htm

Owain

And here is another one (not yours, though): 

"Believe it or not, George isn't at home..
Please leave a message, at the beep..
I must be out, or I'd pick up the phone..
Where could I be?
Believe it or not, I'm not home!"

(from Seinfeld, http://my.ohio.voyager.net/~shoemake/sounds/sein/sein-george-machine.mp3)

OK: there's another possible (though I think unlikely) here:

Lavender Lipstick Lillian's Lovable Linguistics
http://www.witchvox.com/vn/vn_detail/dt_po.html?a=usok&id=2597

Heh :) I came across many strange and amusing answering machine
messages too when I tried looking for the poem myself. Unfortunately,
none of the ones posted are what I'm looking for. Again, the poem I
have in mind is longer, and it lists various reasons why the speaker
couldn't come to the phone. Thanks though, and do keep on trying.