hey!
I had this kind of behaviour from my 6 year old. firstly, I only allowed 1 call and it only came if she could prove that she had been good enough to use the phone. I let her dial, then spke to the responsible aldult first then she had a chat herself, with her friend. that really helped me get to know other Mums, and to identify what was going on in the peer group.
now, the meanness. i tried everything, but this helped and it did work. i got some postcards & sticky stars. then each week, I made three rules for the 3 'R's - respect for yourself, respect for other people, and responsibility for your own actions.
I wrote the three Rs on one side, and a grid on the other side of the card. the grid had my three rules, be kind in class, be polite, eat your lunch (for example) then I made a column for each day of the week, and finaly we agred a reasonable reward for getting 5, 10 or 15 stars in the grid in one week.. then I spoke to the teacher and got some support, and every time she met one of the criteria in the grid, she got a star for that day. it's a mobile star chart really, but believe me, it really worked. you could make one of your rules for the grid, be kind to 9the little friend) each playtime. and get the teacher to ask her the question. It takes a week or two to make your little one understand that you mean business - wait till she doesn't reach your star target & fails to get her reward, you'll see how good the system is then!
good luck,
debs