Clients that won’t answer

What do you do when your clients won’t answer the phone and at the next court date the court is not going to grant any additional continuances? I have multiple cases like this where clients are facing revocation of their probation and the court will be resentencing them at their next court date and they won’t answer the phone, so they are not advised and mitigation has not been collected. They are felonies too so substantial likelihood they are taken into custody that day. It makes me feel like shit and filled with anxiety when I can’t be prepared.