Regulus stared at Sirius, rendered momentarily speechless by his unexpected vehemence, and he didn't know how to answer. Sirius' reaction seemed like a good thing - a sign that he cared - but as Regulus gazed at Sirius, he realized he didn't know Sirius well enough to predict how he would react to the truth. He didn't trust his mercurial brother's reactions enough to be honest; instead, he had put on a front to avoid placing anything that might look like an expectation on Sirius and avoid appearing to need anything from him.
"Yes, Cairo is home," he said at last, unable to deflect such a direct question. "As for calling you, I simply didn't think about it. I'm sorry."