You can stop the vehicle. Which (hypothetically) also serves to have you closer to the phone that may be findable and/or salvageable (if the phone can ring). Chances are not high, obviously.
If you go over 100 kmh, that means ~30 meters per second. Even with instant reaction time to the watch ringing, then you having a look at it and start breaking in a safe manner, you are already way out 100+ meters.
A lot of people will walk more than 100 meters to have an ice cream worth $3. A phone worth $1k+ (stupid), might be worth a stroll. That of course depends on the road and safety etc, I only mention this last point to save you using it as a empty retort.