There's something very weird there. But I think the "iPhone" must be a red herring: if we are talking cellular voice calls then it makes no difference at all whether the call is from an android, an iPhone or a landline, the interaction is between your phone and the cellular network and the source of the call is irrelevant. So that has to be a coincidence/you reading too much into what's happening.
When you say "keeps ringing", do you mean that you can't answer or that it continues to ring after you have answered?
Either way, I agree with Danny: assume it's a glitch on your phone and try just rebooting. If that doesn't work, try clearing the cache of your phone app (or system cache, since I think the S9 uses an old-enough internal architecture that it still has one of those).