Custom data can be passed from a SIP call to a Studio flow execution using User To User information headers. Twilio will prepend the UUI header with SipHeader_ prefix in your webhook request to your Studio flow. This data can be read in Studio using the variable:

{{trigger.call.SipHeader_User-to-User}}

Other custom headers can be read by Twilio if they prepend a X- in the name, for instance X-TestHeader: param1=value1;param2=value2;param3=value3 They can be read in the flow as:

{{trigger.call.SipHeader_X-TestHeader}}