Well, for the string, I would suggest converting the curve into a soft-body curve, add springs, ADD SPRINGS AGAIN to make it slightly more stiff, then extrude a circle around it to give it a sense of "thickness", etc..
Make the String "Geometry", the charm, AND the Phone a PASSIVE rigid body.
Select your string softy, and your charm, and Add gravity to taste, or simply attach them to an already created gravity field. (if you have one set up already.)
This way, the phone still acts as a "parent" chain-link, like it should, and as a passive rigid body, it should not allow inter-penitration (sp?) either.
Tweak the goal weights on the string to get that "elastic, or cotton-string" feeling of secondary motion.
You may have trouble when it comes to your character OR mainly whatever your phone, string or charm interacts with if they cannot be a passive rigid body.
The rigid body sims automaticly handle the anti-penitration for you, but if they run into something that is not, then they will just not see it at all, and they may just fly right through it.
For the character animation "picking-up" part, I would ask about that technique in the animation section.
(never tried that yet, hehe...)
You "may" just have to watch your charm a little bit, and key it at times when it may collide with something you don't want it to.
Good luck!
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS
izzylong.com