This is BIG trouble. My iphone alarm clock doesn’t work for me, so i can’t do WILD, FILD, WBTB, or anything else that requires to wake up in the middle of night.
I’m not sure how loud iPhone alarms are, but I would suggest a much louder alarm clock. If you need something really loud, try using Link to vendor removed 
one, which I have. 
Although if you can help it, then it’s really good to not use an alarm clock. Elegantly fading out of sleep is really nice… 
Mantras can help wake you up early pretty well, even for WBTB, so I’ve heard.
Oh, whoops. I didn’t realize that the link could be interpreted as an ad 
You should try MILD and fall asleep saying “I will wake up after a dream” or something along those lines.
When I was little I would fall asleep telling myself to wake up at 6:30 for Christmas and it worked surprisingly well. I use it today to wake up after dreams.
The iPhone has a few different sounds you can use for the alarm. Back when I had one I used to have to switch it out from time to time as I got used to them. That might help, but like the others have mentioned using autosuggestion is a great tool. Set the intention to wake up and give yourself a specific time, such as “I will wake up in 5 hours from right now.” Imagine yourself doing, make it clear that it will happen.
It’s like telling yourself “oh, I’ll get the chicken out of the oven in an hour.” Just make it something that is going to happen and you are going to remember. The human mind is very powerful, and if you can get autosuggestion down it will really help in your quest for LD’s.
I’d suggest you read this article and its follow up which is found here. Great guides on setting intentions.
I’ve noticed the same problem with my iPhone alarm lately 
Even when I change it to a different sound, I always seem to get used to it too quickly. SO frustrating lol
I actually just went back to my actual alarm clock, which I had previously not been able to use (for the same reason, getting used to it) but I guess with getting used to my phone, I lost the familiarity I had with my alarm clock, because it seems to be working quite well now 
(this alarm clock is very very loud too lol, which was terrible once I got used to it, because then I would wake up the rest of my family (all sleep upstairs lol) in the middle of the night without waking up for a second-- they weren’t too happy XD )