Is it permissible to use the dedicated phone credit from one's job, which is intended for calls and not fully utilized for work, to pay off personal debts by transferring the credit?

The ruling is based on the condition set by the company granting the free credit. If it did not stipulate anything, then there is no harm in selling the surplus beyond one's need. However, if it stipulated that it must be used only for work, then one must adhere to the condition, and it is not permissible to transgress it.