Outstanding question. After all, I know I wouldn't prefer my calls to end and then have to input my password again just to get back into my phone.
Now, I do need to ask if what you're referring to happens only when you receive a phone call? I say that because if you phone is idle and you receive a call, once the call ends, no matter which party ends it, you'll be taken to the lock screen since you weren't already in the phone originally. If however this happens after you're already in the phone and you call someone, then we'll want to explore troubleshooting options.
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