Is it permissible for an employee to use a company phone line without the employer's knowledge, as compensation for the employer's failure to adhere to agreed-upon salary increases?

The fundamental principle is that it is impermissible to take the telephone line without the employer's knowledge, due to the general evidence on the obligation to fulfill trusts. However, if there was a binding contract, and the worker performed his duty and demanded his right, but the employer denied it or withheld it unjustly, and the worker was unable to prove his right or obtain it through a legitimate means, then some scholars permit him to take his right without the employer's knowledge. But if the promise of an increase was verbal without a contract, or the contract expired and the employer wished to change its terms, then he has the right to do so, and the worker has the choice to accept or reject.