I just wanted to post this since I haven’t seen any other information regarding the topic. We recently installed a new VOIP phone system at a client and they were complaining that after transferring a call, the person on the other end was unable to hear the person that was calling from our system. It appeared that the SIP messages were crossing between the server and the two phones.
The issue turned out to be that they were using the Transfer option instead of the Blind Transfer option to send a call to another phone. They would press transfer, dial the other extension number and then hang up as the phone was ringing. Sometimes this worked and sometimes it didn’t.
There are really no ways to prevent this other than making sure your customers are educated on the basic functions of the phone. On the 331 phones you can change the default transfer method to Blind but this option does not apply to IP 550s or above.