the termination notice has to be done in written (and signed) form. Thus, the termination via phone is not valid.
Please give her a termination letter ASAP in order no to cause further damage. Get a witness to be able to prove what has been thrown into her mailbox or sent via certified letter.
If it is a normal renting contract for living space, this is what applies by law:
If you manage to make her receive a termination letter until 3rd of november (third working day of the month) the contract will be terminated by the end of January.
It is illegal to stop paying the rent because of the deposit. The deposit has the purpose to protect the landlord from other damages after the termination of the contract.
Theoretically the landlord could sue you before the termination of the contract in order to get the rent and will win the lawsuit even if the money on the deposit would cover the rent.
However, if you get out anyway and the state of the apartment is rather OK, she may choose not to do so.
I am sorry for the bad news.