How do you capture the exact duration of the telephone call? Calculating through remote_participant events gives us a higher number than the actual duration.

You can use the Analytics API for this: https://docs.livekit.io/deploy/admin/analytics-api/

Does it also include the ringing/dialing time in the metric?

That API will give you

joinedAt

and

leftAt

for the SIP participant, which will not include dial time.