"What the customer wants most is proof that you are taking responsibility for their issue and a clear pathway to resolution. Even if it will take days to fully resolve, this principle will buy you time and earn back trust. Say exactly what you will do. Do exactly what you say."
Example
A customer is angry about an unresolved bug. Instead of "I'm sorry, I'll look into this," say "I just messaged our Head of Engineering for status. By 4pm today I'll send you what I learned about priority and timeline, plus any workarounds I can confirm from testing." Then by 4pm, send it - even if the answer is "no timeline yet." The fact that you delivered the update you promised on the time you promised is the trust rebuild.