Implement SIP 603+ (ATIS-1000099) support for FCC analytics-based call blocking compliance.
Detection:
- Detect incoming 603+ responses by checking "Network Blocked" phrase
and "v=analytics1;" in the `Reason` header text
- Set `sip_603plus_reason` channel variable on both legs for CDR visibility
Passthrough control:
- `sip_603plus_passthrough=true`: forward 603+ phrase and Reason header
- `sip_603plus_passthrough=false`: strip `Reason` header, send clean `603 Decline`
- Not set: existing behavior preserved
- Works independently of `disable_q850_reason` for selective forwarding
Dmitry Verenitsin