Hello & Welcome to Just Answer,
I'm Greg and will do my best to assist you today.
Sorry I didn't see your question earlier and to hear of any issue you are having with your phones.
To help you with this issue, may I ask:
1) WHERE are you trying to change this code?
2) What do you see on the DISPLAY - after entering MENU #306?
I see it has been a few days and I haven't seen a response on this page, so I just wanted to follow up to see if you have any more questions concerning this issue OR if the issue was resolved?
I look forward to any further questions or results that you may have.
Thanks, ***** *****--. .-(im)