Twilio Programmable Voice, Client, SIP Interface, and Elastic SIP Trunking calls are all rounded up to the nearest minute for billing purposes. For example, an automated one-way call of 1 minute and 20 seconds would be billed as 2 minutes.
Similarly, a two-way call connecting two different Call SIDs for 1 minute and 20 seconds would bill you for two different calls of 2 minutes.
Notice: As of February 28th, 2019, calls to China have a different billing scheme. For more information, please see Calling Limitations to China.