This one is easy:
Just tell her you do it, because you want to focus 100% on her.
For real, if I have my phone out, it will be face down. Otherwise, it will stay in my pocket/bag. Heck, I even find having your phone on the table is a bit disrespectful to the people you are with. Call me a boomer, but when I'm with someone I want to focus on them. If someone absolutely needs me, they'll call and then I'll notice, the rest isn't so important I'd need to answer instantly.