secret telephony

例句：

1. 1.They used secret telephony to communicate with each other without being detected.
   他们使用秘密电话进行通信，以避免被察觉。
2. 2.The government has been accused of using secret telephony to cover up their crimes.
   政府被指控使用秘密电话来掩盖他们的罪行。
3. 3.In the old days, people often used secret telephony to send messages across great distances.
   在旧时代，人们经常使用秘密电话来跨越长距离发送信息。