Transfert button disabled on Lync 2010 client

Hi,

We user lync 2010 client with polycom cx600 on a entreprise voice edition. Sometimes, an internal user can't transfer an external call because the transfert button are disabled on both windows  (with 2 calls).

She uses the last client version, but it's hard to reproduce it to open a case on microsoft support.

Do you expirement it also?

Thanks

Marc