Not all users sign in to their PCs with Domain credentials. For those that sign in to a local user on their PC it is a nuissance to enter your username and password every time you sign in to Office Communicator
I found a regkey that enables the Save Password checkbox under the Password field.
SavePassword created as REG_DWORD and the value should be 1
Next time you sign in, check the box and from then on it works just like you are signed in with Domain credentials.