Why can't I verify my mobile phone device?

Thank you for the reply and sorry it has taken me so long to get back to this. I have confirmed that I am still unable to verify my device. Here is the additional info you requested:
• Country: USA
• The name of your mobile carrier: Sprint
• I am sending the code to 40404

Here is some additional info you did not request:
- I entered my device number in this format: +19195551212 where 5551212 is used in place of my actual number
- If I manually enter 40404 in the To field on my phone's messaging application it resolves that to "Twitter". I have sent the code using both that method and also by adding Twitter to my contacts list with the number 40404 and initialing a message through the contact. ( Delete or Edit)

Thanks for the extra info, John Holtman. Now I can give you a couple of links:

• There is a known issue with Sprint (USA, 40404) who sometimes block 40404 for some users. This is most likely your problem.
Ask Sprint customer service to unblock shortcodes, specifically 40404, for your mobile account.

• Note that currently (as from late Oct. 2008), SMS text delivery is unreliable or nonexistent for some users in the USA or Canada on various mobile networks. This might now interfere with your registration process when you try to send the verification code.
(Twitter are already working on this problem for cases involving Canada or Verizon USA.)
If you think you may be affected by this, post in that thread along with the other users. ( Delete or Edit)

Having your phone verified is a great step in the right direction, though!
Are you in the USA using 40404? Who is your mobile carrier - not Verizon, by any chance? ( Delete or Edit)

The required format for cellphone numbers is country code, area code, number (with no spaces).

For example: +12125551122, where
+1 is the country code
212 is the area code
5551212 is the phone number

If Twitter asks you to send to +44... then it's possible that the +1 country code was missing at the time you entered the info (which led Twitter to believe that you are not in the US). ( Delete or Edit)

Eridanus, do you suppose Drew has the username1 problem? Since he says Twitter responds to his phone...? ( Delete or Edit)

mdy - I hadn't thought of that; it's certainly possible.
Here's the link for the username1 problem and fix, just in case. ( Delete or Edit)

the username1 problem and fix took care of my problem. Thank you. Thank you. Thank you. I was getting very frustrated. ( Delete or Edit)

Drew - Thanks for letting us know it finally works: that's great news :-)
mdy - Thanks for the decisive tip 8-) ( Delete or Edit)

Mermaidlost:
Since you're in Australia, the message

Note: We currently don't support sending SMSes to this number. You may still verify your phone and update Twitter, but we'll never send you a message.

is not an error: it indicates that Twitter SMS text messaging outside the USA (and parts of India) is a strictly 1-way affair: SMS to Twitter should appear as updates on your http://twitter.com/home page - but (for reasons of cost) Twitter will never send any SMS to your phone.

You might like to ask another Australian how to enter your number in full international format (i.e. the way someone from another country would call your number - something like +61XXXXXXXXXX - with no spaces, leading zeros or other extra characters). The international code for Canada is +61.

If you entered your number wrongly, just click the Delete and start over? button and enter your number correctly.

Then you'll need to send the verification code (which looks something like wtpmjg) by SMS to Twitter's +447624801423 number.

Once your phone is correctly set up and verified for Twitter, the message on your http://twitter.com/devices page will change to something like:

Note: We currently don't support sending SMS to this number.
Mobile Phone: +61XXXXXXXXXX

where +61XXXXXXXXXX is your mobile phone number in full international format (no spaces, leading zeros or other extra characters).

If you need 2-way Twitter to and from your mobile phone, an acceptable interim solution might be one of the paid and unpaid alternative ways of receiving Twitter updates on your phone - a thread which includes plenty of (hopefully useful) links.

Twitter say they are negotiating national numbers for 2-way SMS text on a country-by-country basis - but such negotiations are apparently complex and protracted, and we're unlikely to hear much more before the first new national Twitter number actually goes online.

Read some of the more recent posts in that thread, too. As I'm from the UK but based in Germany, I'm interested in any new developments, and I (and hopefully other Twitterers) will post updates there from time to time. ( Delete or Edit)

Don't forget the initial + sign! Replace the initial 0 with +61 (the international code for Australia) as follows:
If your number is: 0411654321 then Twitter expects you to enter: +61411654321 on the http://twitter.com/devices page.
If it still doesn't work, then a successful Australian phone twitterer would be the best person to help you.
I certainly agree that Twitter could help by including a dropdown list of the international codes on the http://twitter.com/devices page. ( Delete or Edit)