Such word form don't used in Russian language, but there are different ways for to
convey the meaning and it depends on you relation to person (to whom you say that), context (style of the rest your speech) and conditions.
For example, employee of a company say to you like a client: Вы можете звонить в любое время. But i want to pick up a girl, i end my speech with: Звони, не стесняйся. But to friend i say: Звони, если что. etc.