I'm not familiar with the specifics of iOS's interruption of other apps when calls come in, so there may be a simpler solution somewhere in the iOS device API. An end-around solution might be to keep the call alive on a central server that acts as an intermediate switchboard (a third caller, so...
An andoid app when published on Google Play automatically get SDK integrated.
I'm not a lawyer but I've dealt with similar matters in the past. Technically the author of the post, but you as the publisher you can be liable for the content specially if you don't publish the original credits.
That sort of idea was most successfully implemented by a company called Quirky.com (https://www.theverge.com/2015/4/24/8488531/quirky-invention-powered-by-quirky). Quirky got a ton of investment and grew very quickly. I had meetings at their office several times, and it was a really fun environme...