This seems like your carrier issue, Contact your Carrier (Cellular Service Provider) The agency who issued the SIM to you.
Call Failed or Dropped Calls / Disconnecting
In telecommunications, a dropped-call / Auto Disconnecting Calls / Call-Failed Muted conversation are mainly due to technical reasons / fault at the carrier's end. At times the calls are cut off before the speaking parties had finished their conversation and before one of them had hung up (dropped calls or Call Failed or Auto Disconnection or muted calls).