In this sentence, we need to use the past tense for both verbs to indicate that the actions happened in the past. 'Called' is the simple past form of 'call', and 'was going' is the past continuous form of 'go', which is used to describe an action that was ongoing at a specific time in the past. The phrase 'just as' indicates that one action was happening at the same time as another, which is why we use the past continuous for 'go'.