I'm using the clickatell.com platform to send SMS from my app to my user.
It's really just to get a one-time-pin. Nothing more.
It works just fine for everyone except for some (random?) users. Some of them are iOS7 others are iOS8, so I'll assume it's not version related.
Situation :
On clickatell : The SMS is clean, has the right number, shows the right provider and that provider is successful in delivering SMS to my other users. Everything is tagged as delivered to handset and all is wonderful on that side. They send their messages from +## xxxxxxxxxx regular numbers, not "8603" or something short like that.
In the code : He uses the same code as everyone else and the code returns no error (otherwise an alert would show). So we also "assume" it runs smoothly there. All of my users run the same version of the app and use the exact same code no matter what (no preferences/special users/etc.)
The device : They have non-jailbroken iPhones of version going from 7.1.2 to today's latest. The same device asked for the pin several times without getting it. We tried a week later without changing anything and the SMS went through. He is obviously not on Plane mode and has data active as well as network. We've tried on plane mode or in a faraday cage (actually it was a bag of crisps), we get an error message just like it should.
The testing : I've tested on iPhone 4S -> 6 on iOS 7.1 -> latest. Works fine. Every. Single. Time. I've also contacted clickatell this morning but they were of no help. I've asked if they've had this kind of situation before though in hope they can point me in the right direction and... they couldn't.
So, finally, we're getting to it : My question.
What could possibly prevent a user from receiving an SMS from clickatell.
I have no idea where to look. I could show you code but it's pretty much a copy-paste from their documentation except it has my access keys...
Are there settings? apps? SOMETHING??
EDIT 1 : Clickatell answered my email and was (to keep it short) of no help.
EDIT 2 : I know this is a horrible question to investigate because you have nothing to work on ; neither do I. I'm really looking for hints/directions (or even a solution) but I don't expect you to type a magical line of code that will fix everything. Really if you have an idea just shoot it in the comments or an answer, I'm not here to downvote if it's not complete enough, I'm just here to get those SMS to go through...
When you send a message via clickatell, the API returns an API message ID (a unique reference number for every message submitted). Could you please give me a couple of API message IDs for the failed messages, and I can escalate this for further investigation (with the network operators, if necessary).
Disclaimer: I work for Clickatell