Improve SIP call events that are currently difficult to use

[atsakiridis]

Some issues with current API:

• When making a call cannot differentiate between BUSY and DECLINED/REJECTED. They both send the same event in the App which is onDisconnect
• We cannot tell if the caller is CANCELing the call directly. Instead we need to infer that infromation.
• When a call is normally hung up after talking, again onDisconnect is sent.

Proposed solution:

• Clearly separate BUSY, DECLINED/REJECTED and CANCEL, either by adding an additional field in onDisconnect, or by providing separate callbacks (right now we have additional callbacks defined but we don't use them)
• Make sure that Connect properly differentiates between BUSY and DECLINED/REJECTED and that the solution will work E2E

Related to https://telestax.zendesk.com/agent/tickets/34334

[marca56]

Can't we use SIP response codes like 486 busy and 487 cancelled?

[atsakiridis]

@marca56 we 're already using those SIP response codes, the problem is that we don't properly expose them to the higher level API, so that from user perspective things are more clear