Saturday, March 3, 2007

Error Handling & Pay Phones

It's not enough to handle all error cases; you have to respond to them properly as well.

While waiting in the Ottawa International Airport, I placed a call to my sister from a pay phone. The number is local, so I entered the seven digits. The phone's response was to play a recording for me: "To dial this local number, you must enter the area code." I tried again, entering 1+<area code>+<local number>, what I had assumed to be the only way to dial a number with an area code. To this, the phone played another recording: "This is a local call. You must not begin the number with a one." Finally, I dialed <area code>+<local number>; this worked.

In both failed cases, the phone system detected an error and alerted me, as well as suggesting the solution. So it did one thing right, and two things wrong. The suggested solution was not clear enough, but more importantly, the phone system knew the number I was trying to connect to; it should have adjusted the number and connected me, without notifying me of any perceived errors.


  1. Haha, yes, because people love it so much when software automatically tries to figure out what we really meant and do it for us.

    It looks like you're writing a letter.

  2. This is a bit different than clippy. Do you always dial 10 digit numbers when making local calls (I doubt this even works on most private telephones)? Similarly, when you enter a URL in a browser, do you sometimes leave off the protocol?

  3. Stuart MacDonaldApril 3, 2007 at 1:41 PM

    The phone system did everything right, the error is on your end.

    1: The first error message told you exactly how to dial the number you
    wanted, you then made an incorrect assumption and did something else.
    2: The second error message was also correct, correcting your mistake
    from 1.

    To the suggestion not being clear enough, usually the message is
    "This is a 10 digit local number, you must enter...". So it could be improved

    To the automatic connection: a) it is not possible to know which number you wanted. There are multiple local area codes, and hence multiple
    possible numbers that you could have wanted. b) Bell et al use this error
    message system to modify the behaviour of the people dialing. They
    don't want to auto-connect (in the rare case of an unambiguous match)
    because it will bypass that.


  4. Good point about multiple local area codes, Stuart. IIRC, Ottawa has only the one, so it should still be possible to figure out where I was trying to connect to. Residential phones don't require the area code, so why should the pay phone?


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