I'm not a professional teacher but I can assume that in this case you should form the imperative according to the verb звать, which, if added a prefix, can form many derivatives - вызвать, назвать, позвать, призвать, созвать and the imperative for all of them will be the same, only the prefix is different (вызови, назови, позови, призови, созови). However, the first verb has the stress differing from others. May be it's an exception...I'm not strong in the theory Let's wait for another reply