It's just a terrible idea. I'm guessing it took less than a minute to come up with it, and you spent that time thinking of reasons to support it and almost no time thinking about the problems with it. It's just that bad.
Some obvious problems:
1. You didn't consider the most popular smartphone OS at all.
2. Not everyone has a smartphone.
3. Not everyone who has a smartphone knows how to use all the apps on it, or can figure a new one out in a stressful situation.
4. How exactly do you think someone who's actually being threatened would prove it to the police over a video call (reveal their hiding place to take video of the perp with the gun)?
5. Don't you think the SWATter (or whatever you call them) could fake a video call using some accepted service, like they already fake phone calls?
6. What if there's no data at their location, or the signal is too poor for video but good enough for voice (like my parents' basement)?
7. Etc. (I could go on, but I think I proved my point)
Most of them are dealbreakers. A few even show that your solution won't even properly address the SWATting problem it's ostensibly meant to solve, even ignoring the bad tradeoffs.
I'll grant you it's a step, just not a step forward.