I have a web app which will relay messages from my desktop applications (which is a workflow application for my client).

A typical message would be - 'you have booked your pc in for a service and we have agreed a fee for this work. But, we found a RAM stick needs to be replaced and we need your permission to do this for this new price. Please reply to this message with a 'y' or 'n' (yes or no).

This would be sent from my web server app to the Users registered mobile.

The mobile User would then reply 'y' or 'n'.

But is this a reliable way of doing things? What if the user gets confused by the message? What if they accidentally sent a 'y' instead of a 'n'.

Should I rely on this? Should I send another SMS to the mobile user for confirmation?

Would it be better to have a link in the SMS that directs them to the mobile browser where they have to enter a code and tick a box and then submit these details back to my server.

First time I have done anything like this and normally I try to educate myself via Googling but have not found any thing out there.

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    Are you asking if this is okay from a legal perspective in terms of getting permission for work, or some UX aspect of it? If you're asking if using an sms for confirmation is good UX, then you should reword your question to focus on that.
    – JohnGB
    Jun 10, 2016 at 11:05
  • @JohnGB Good point(s). I would need to work out a digital signature (somehow) for authentication to take place I guess. That is agreed before hand when user books in their PC. I my main focus of this question though was to establish the best way of replying 'yes' or 'no' to mitigate a mistake by the user when replying their wishes. (I need to Google digital signatures now :) ) Jun 10, 2016 at 11:07
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    It's not about a digital signature, it's more whether you can consider a simple message as user consent. I suspect it depends on the agreement you have with them, but you would have to clarify this with a legal expert. Other than the legal aspect it's not clear to me what you're asking here.
    – JohnGB
    Jun 10, 2016 at 13:13
  • @JohnGB hi thanks for taking the time to post. I have just consulted a legal expert and now I know where I stand and what I need to do. It does address my poorly worded question and I will delete in a few mins so u can se my 'thanks' :) Jun 10, 2016 at 14:30
  • Think about security for a second. Your only way of telling each reply apart is by the originating number (caller ID), which can be spoofed easily. This is a bad idea. Sep 9, 2016 at 8:35

5 Answers 5


You should never rely on SMS.

Delivery is not guaranteed, and you have no way to determine whether or not it took place. Additionally, SMS is not encrypted, and from a security standpoint, it is not advisable to deliver any kind of private information over SMS or rely on receiving unmodified responses via SMS messages. These factors should precede UX considerations.

  • But if they reply, surely they received it? Aug 3, 2017 at 8:25
  • Thanks but I am not bothered about encryption and if I was I would encrypt the message before sending. Whilst I can get a code back from the SMS gate way to say the message has been sent you right that there is no way the SMS has been received and read Aug 3, 2017 at 8:31
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  • Well you can go to the n'th degree and there are far more established companies relying on SMS for many level of their services, like Apple, British Gas, DPD. I'm not saying these issues should be ignored, but you have to weigh up the level of risk, otherwise you may as well sit in your disconnected corner and put your tinfoil hat on. Aug 3, 2017 at 8:52
  • @UXfrom12, do you have examples of any of those companies sending sensitive info or authentication data via SMS?
    – Tim Grant
    Aug 4, 2017 at 20:53

Let's leave the legal discussion aside for a moment.

So, is this a good UI? Simple answer: Probably yes. It's simple and if there is a default option if the customer does not reply at all.

The more complicated answer is: Do you trust an SMS asking you to reply in order to pay for something? Probably not. And, would you like to pay that extra fee for sending an SMS?

In this case, the best solution is to call the customer and explain what has happened. A nice call will probably increase the chansers of a positive answer.

  • hi, thanks for posting. There is a contingency to handle a non-response in a period of time. The problem my client has at the moment is that they DO call the customer and the work and costs are agreed and in some cases the customer goes into the shop and argues about the cost. Legally this leaves the retailer open. Sometimes it is not convenient for the customer to go into the shop and counter-sign the new work. So, an SMS system is being proposed. Jun 10, 2016 at 15:03
  • The legal advice I got permits this and my original question does pertain to the reliability of it all but after a period of time the customer would be contacted by telephone. The costs of SMS is negligible as I buy in bulk from a service provider. The only issue i have now is what to respond with if affirmative . i am considering asking them to send me their postcode as a way of verifying they want the work done as opposed to a 'y' or 'n'. if the post code is not matched they will be either re-sms'ed or/and called directly to clarify. But thanks for your time it is much appreciated :) Jun 10, 2016 at 15:03

All legal talk aside, "yes" and "no" would be better UX than "y" or "n"

It's clear, concise and requires a little more work than "y" and "n" but would drastically reduce the potential for a user making a mistake.

I received one today from my dentist and it had an opt-out that said ReplySTOPoptout.


It is a design solution that could work. 2-step verification/2-factor authentication for logins commonly uses phone and phone apps. I confirm appointments via text. However, is it the right solution? Do people feel comfortable using their phone for additional charges (the RAM)? It is not a common model, you would do well to get some data with your customers - if this is for a repair store, then ask people if they would be interested in opting into something like this.

On top of that, you said "reliable". When you say reliable, the UI design is not just about the customer comprehending and replying "Yes" or "No", there is also a lot of other things that must happen. The customer must receive it and receive it in a timely manner. If you have customers in areas like China, SMS is not very reliable in my experience. If your customers work in settings without easy access to SMS, that is another problem (for example, I have family that work in a government building without phone signal). However, it could still be good for a good portion of your customers. You would need to answer that trade-off.

For Googling, it may help looking into "2-factor authentication"/"2-step verification" if you want to learn more about SMS as a method for reaching your customer reliably. Also, "text opt-ins"


I agree with @ilias's answer. A simple call would be nice, especially when it comes to asking the user about extra services for extra cost. Maybe, a message after the call to confirm the user's verbal response would solve your problem. In cases where the user forgets, a message could be sent again in 24 hours (for example) to reconfirm and a call on the 2nd attempt.

  • The point you are missing is the legality of the situation. Customer signs for work to be done. Work to do changes. Therefore original agreement has changed and what the customer had originally signed is now null and void. So forget the call because if the customer agrees for the extra work via phone they can always deny that gave permission for more work to do. Therefore the customer would need to come into the shop and sign a new agreement. By sending SMS instead it will save time and the customer having to come back into the shop. Forget about what is nice or not. Oct 9, 2016 at 7:00

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