Always sucks to have people use their phones on you especially on dates. Having had some experience with girls from this age group (I'm in that age group)

you could say/do:

1. "hey we're on a date here and the purpose is to get to know each other and if the communications on your phone are more important we could always pick this up at another time"

2. "if you really must message someone about something, please take some time to do it all then come back and focus on this (AKA get it out of their system.).

3. inquire, ask about what she's distracted by "Oh that's a famous youtuber? what does he/she make videos about, hey lets go take a photo with them, whats you favorite video from said youtuber."

That's similar to you seeing an actor or minor celebrity you like and wanted to take a photo with him/her while you are out on a date.

Other than these, i think, if they insist that their phones are more interesting than you although you are supposedly their date for the day/night. Excuse yourself, drop any later things planned for that date and call it a night, they don't need you.

##Edit:

Seeing what Alan wrote, i think yeah, using the phone deals with the luls and lowpoints. my guess is that OP is annoyed as it happens very often or maybe even halfway in conversations. If both are silent and there is no communications going on, a 30 secind phone whip out would hardly be out of place. Issue is when my date shushes me mid sentence so they can reply rebecca about how absolutely cute that dress is. (not a true story)

Always sucks to have people use their phones on you especially on dates. Having had some experience with girls from this age group (I'm in that age group)

you could say/do:

1. "hey we're on a date here and the purpose is to get to know each other and if the communications on your phone are more important we could always pick this up at another time"

2. "if you really must message someone about something, please take some time to do it all then come back and focus on this (AKA get it out of their system.).

3. inquire, ask about what she's distracted by "Oh that's a famous youtuber? what does he/she make videos about, hey lets go take a photo with them, whats you favorite video from said youtuber."

That's similar to you seeing an actor or minor celebrity you like and wanted to take a photo with him/her while you are out on a date.

Other than these, i think, if they insist that their phones are more interesting than you although you are supposedly their date for the day/night. Excuse yourself, drop any later things planned for that date and call it a night, they don't need you.

Edit:

Seeing what Alan wrote, i think yeah, using the phone deals with the luls and lowpoints. my guess is that OP is annoyed as it happens very often or maybe even halfway in conversations. If both are silent and there is no communications going on, a 30 secind phone whip out would hardly be out of place. Issue is when my date shushes me mid sentence so they can reply rebecca about how absolutely cute that dress is. (not a true story)