is this your first smartphone? if your screen is off due to proximity sensor for call, it is not locked, it is off so screen does not get activated while touching your ear. Press the power button to switch on the screen. if you are using a cover/tempered glass that covers the proximity sensor that too may cause this.
What the OP is saying is right. sometimes when the call is going on speaker, the phone gets locked and shows the fingerprint icon which doesn't work. we have to expand the call from notification and then it unlocks. Happened to me quite afew times though not after updating to beta 2
