Also, this issue may involve parenting. If you animate an item that’s parented to another item, it will inherit the parent item’s motion. Try setting up your character/phone stage like this, with both the character and the constraint transform under the same root.
The key is to set up your phone/constraint like this in your character/phone scene. Then it will not move relative to the constraint unless you move it. Having set it up like this I can move my character anywhere and the phone will stay connected to the hand constraint.
Remember my comment above about issues encountered when animating something that is parented to something else? Well, when you pull a stage into another stage, everything in the original stage is parented to the “default prim.” The phone is in the parenting hierarchy of the default prim, and yet it’s trying to set its position based on its relationship with the constraint transform, which is matching the bone. What is happening, in effect, is that the phone is adding the transform of “Female_lowboly_MobileStanding_realtime” to the bone transform, and the bone transform already has that transform in it. You are, in effect, doubling the size of the transform.
Do it this way instead: select the transform of your character (which I believe, based on the image above, is called “ManRoot”)and keyframe that in the time sample editor.
In this video I’ve added the stage I made in the video above to a new stage. As you can see I’m animating the character’s root transform, not the stage reference transform, and the phone follows the constraint even with a one-frame move.