Tony - 

He upgraded it so that it had all these new functions on it that you were never going to use and in all honesty, didn't know how to use.

Steve - 

He used it to teach himself how to use the technology in this time

Clint -

He took selfies of himself and his bow and arrows in the bathroom, trying (and failing) to recreate other pictures that had been posted online

Bruce - 

He used it to download some apps with scientific data on it for you to read as he wanted you to be up to date on all the latest things he was working on

Thor -

He didn't get to use your phone as he accidentally crushed it in his hand when he grabbed hold of it

Loki -

He decided to google how to take over Earth which raised a lot of questions at S.H.I.E.L.D when you went into work the next day