T46G transferring calls issue

[george.deligiannis@frontiersin.org]

Hello to all.
I am working for a company that is using the T46G.
The Firmware Version is 28.81.0.90 (updated it last week)
The Hardware Version is 28.2.0.128.0.0.0

The problem is that since last two weeks, the girl at the administration office couldn't use the transfer function with no issues and transfer the calls.
I will try to describe the problem:

administration girl = Maria
Maria is using the T46G
- Client A is calling my company
- Incoming call gets answered by Maria (T46G). She talks with the client A and tries to transfer the call.
- Maria presses the transfer button.
- Maria types the number of Communications office (Comms) or clicks the quickdial for Comms.
- Comms respond to the call of Maria and they wait for the transfer.
- Maria presses the transfer button again, to transfer the call from Client A to Comms and hungs up the receiver.
- Client A's call isn't transfered and stays waiting at Maria's phone (T46G) hearing music.
- Comms get an error in their phone (not yealink) and it says "error 701".

Please do not focus on the error Comms receive (701), since it's a different phone and we tried to do the same with an external call and smartphones.
This transferring problem started two weeks ago without doing anything. In the past the phone was working for at least 2 years with no errors.
Can you tell me what to do or check?
It's very frustrating for our clients to tell them we can't transfer calls and propose them to call directly to someone.

Many Thanks,
George