smart phone screen goes black before I can hit speaker?

I have a zte valet smart phone. some calls I make I want to hear on the speaker and input from the keypad. quite often the screen goes black before I can hit the speaker button. the geek squad has suggested I increase the stand by time in settings. I can't find it. anyone know how I can increase stand by time on my phone? or any other way I can solve my problem?

Lily2017-03-01T06:24:22Z

Same with mine, I'm not sure how to fix it but it stops doing that after a few seconds, hope this helps 💗