When my Mom was sleeping, I added a friend's # to her phone, and titled it to appear as a sister she texts frequently, giving this friend my Mom's #. Friend and I came up with ideas of what she could text my Mom with. I also gave friend (very trusting) the name of my cousin to use to strengthen the prank.
Later that day she does, and Mom falls for it, but it wasn't until night that friend and I both announced the prank.
She used the whole first name portion to add it "[name] - sister". I had to use "[name] - " as the first name and "sister" as the last, so that if she happened to go through her phone list, friend's # would be the first to show up and she'd never know. (I kept trying to think of a plan had she gone down one more to see a duplicate entry, but nothing, never came to that though)
I guess as a + side, they have each other's # now and can text when they want, as they are friends themselves
I almost blew this gag, as during the night I almost called my aunt studying how the name was entered, hit the 'green' button instead of the one above it while on the profile for her #. The # started dialing and as quick as I could, hit the 'red' button to cancel out of it... ~whew~
Was I prepared for a possibility of her saying anything embarrassing to friend? NOPE!

. But again, this friend is very trusting, so there wouldn't have been toooooo big o' deal had anything personal been said.
As a side note, she came up with the idea of saying "I'm feeling sexy and free!" and was going to use it as her 2nd reply, but with my Mom's replies to her, it never worked out
And finally, she plans to keep her phone away from me

. Was she mad? Nope.