Call file mechanism

Hi list,
I have a call file as following and it works. But, I don’t really understand its mechanism.
The SIP/voipbuster is a sip trunk which I set up in freePBX with voipbuster account. And 2874 is one of my extension which was assigned to x-lite client.
When I place this call file in outgoing folder, it is able to dial out my home phone at 001xxxxxxxxxx. However, the Dst in call logs show 2874 or s instead of my phone number. Why sometimes 2874, sometimes “s”?

My interpretation is the call file actually “call” extension 2874 and place a out going call via 2874. If I am right, does it mean any outgoing call has to be placed through an extension. How can I manipulate this call file in order to show my home phone as destination instead of extension number.
Thank you very much.

Channel: SIP/voipbuster/001xxxxxxxxxx
MaxRetries: 1
RetryTime: 60
WaitTime: 30
Context: outgoing
Extension: 2874
Priority: 1