I just test-drove the Vanido voice ear trainer iPhone app for a week and have decided to fork over the $70 to keep it.
It presents a different set of exercises every day - nothing fancy, just regular scales and intervals up and down your range. It's game-ified so you get points for doing the exercises; it reminds you that it's time for you to practice; and it keeps track of how many days in a row you've practiced.
Each practice takes about 15-20 minutes. I think you have to wear headphones to use it; I wear one earbud with the app playing its pitches and leave the other one to hear my pitch. I don't know if this is going to change as I get more into it.
I'm a beginner and I think in the last week that I've been using it I've gotten much better; my neighbour, who can hear me practice, spontaneously commented that I had.
Although built for the human voice, it seems to cope OK with the inhuman 5-octave range of my extended Moog Etherwave.
My music teacher boyfriend says he thinks it's well worth the $70 a year.
Here's the App Store page for it
* https://apps.apple.com/ca/app/vanido-learn-to-sing/id1130249200
And here's a review:
* https://www.technologyreview.com/s/608951/can-an-app-really-teach-you-to-sing/