There is a problem with the X_UseRefer option not working as it should in some, but not all situations. Apparently it's an issue that shows up primarily with certain versions of Asterisk, and my gut feeling is that it involves a timing issue of some kind. Obihai customer support did push some beta firmware to my OBi202 which fixed the issue, but as far as I know this fix has not yet been rolled into any version of the release firmware yet (however there's been no new firmware releases since they made that fix). If someone has a real need for this feature to work and it's not working now (check first, because apparently it does work with some installations), and you have an OBi202, you could request that customer support push firmware version 3.0.0 (Build: 3089MD ) to your OBi202 (I have no idea whether they have a version of the OBi1x0 firmware that contains this fix).
If it's not working, what will happen is that when you attempt to transfer a call, it will act as if the called endpoint is busy and handle the call accordingly (by returning a busy signal, going to voicemail, going to the next number in a follow-me, etc.).
Hopefully Obihai will include this fix in the next full firmware release, because it definitely solved the problem for me.