- The condition where a telephone is in use. This state occurs, for instance, when the handset is removed from its cradle, which releases the hookswitch, which in turn occupies an available line. Also, when another device, such as a fax, is in use. This contrasts with on-hook, where the telephone or other device is not in use. Also spelled offhook. Its abbreviation is OH. Also called off-hook state, or off-hook condition.