I've connected a brand new ATA-186 to a customers UC520 today, the extensions showed up nicely and after giving them their proper extensions the worked like expected. After a couple of test calls it was clear that about 4 out of 10 of incoming calls came back busy. This was regardless if the calls came from the outside or another internal extension. It was completely random and sometimes 10 calls in a row went through and sometimes 7 calls were rejected with busy signals.
There's five other ATA boxes on this system that doesn't have this problem.