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Since you really seem to care for your users and think through your interface, I have one suggestion: Users should never have to manually type arbitrary codes! Instead, just generate a code from a freely available English wordlist: Your activation code is "Large Sinister" Of course, interpunctuation, capitalization and spacing should not matter.

Yes there are examples of how to solve your issue. pwgen has a list of ambiguous characters: "B8G6I1l0OQDS5Z2" (it's in this file if you can read C code). Another code-snippet is here, in php this time. The former approach is using a "blacklist" approach, the latter instead a "whitelist" one.

First off, those who want to cheat will find a way to do so even if they have to vote in person. All you can do is make it as hard as possible because even with SMS verification people can have multiple mobile numbers, multiple Google Voice numbers, and nearly every GMail account can send & receive SMS with a unique phone number, too. So the biggest ...

In general, assigned passwords should avoid ambiguous characters. An example is Microsoft activation codes; they do not use the letter O because some people might type a zero (0). Similarly, you should trim your allowed characters to remove any ambiguous or confusing ones from the set of possibilities. For example, this is the character set I use in ...

Microsoft has has to deal with user-legible activation codes for a long time. I suspect they have put some science into their decisions. I had a pile of activation codes from work to analyze. Here's a faked example code: V3MKH-7GMWJ-PHRWW-Q9RD3-M84FR Firstly, all letters are UPPERCASE. This move alone should eliminate quite a bit of confusion. (I ...

Base 32 encoding is a standard defined with this purpose. There are several variants, but all of them try to avoid ambiguous characters. This is the rationale from the article linked: Base32 is a notation for encoding arbitrary byte data using a restricted set of symbols which can be conveniently used by humans and processed by old computer systems ...

This may or may not be considered "keeping the same layout" but what came to mind immediately was this: add checkboxes for SMS notifications next to each relevant entry. The space is already there. What you have here makes it seem as if once you enable SMS notifications, you will get those notifications for all of the listed reasons. If that's not how the ...

I think you shouldn't teach how to send sms 'in general'. It's enough to mention something like 'Refer to user manual of your cell phone on how to send SMS'. The main reason is that if you provide some service - teaching SMS sending - and you don't succeed, your customer has reason to be dissatisfied you. Especcially when your instructions are misleading in ...

One more vote (if I'd have reputation) for making the string itself easier to remember, like WCode or Ben's answer with "Large Sinister". On the international and technical side of things, the character set used should be http://en.wikipedia.org/wiki/GSM_03.38

I believe that SMS verification isn't much of a good idea since not many people have mobile phone with SMS capabilities. Also, many people are afraid that they might get spam (even if it stated that they won't get spam!) It would be best to ask for a address (building number, street name, and ZIP code) that is used for verification purposes only because ...

It depends on the prize being offered. For something minor going for SMS verification might well be over the top. For a major prize then it could well be appropriate, and I can't see people objecting if you are offering something to the value of a thousand dollars or more. Perhaps you should only use the number for a one shot verification and then discard ...