We started to use BigAnt messenger in 2012, very satisfied with it. It allows group discussion and broadcast message, which makes collaboration more convenient. Worth trying.
Since we used the bigant messenger, the office communication is largely improved. Another office messenger named CD Messenger is also good, but this one has a big discount here, so I purchased it.
We use it in our inner office and it makes our office communication more efficient. Hope it will release an iPhone app for this product.