Sinch Elastic SIP Trunks are 603+ compliant. If you send a call which is blocked by the far end network they may return a "603 Network Blocked" message. Contained within this message will be a reason explaining why the blocking occurred and often a URL or resource which can be followed to request that the block be removed.
Example of a 603 Network Blocked response.
Reason: SIP;cause=603;text="v=analytics1;url=https://www.t-mobile.com/responsibility/consumer-info/additional-info/call-blocking-redress";location=RLN
It should be noted that while the regulations refer to this as "603+” there is no actual “+” sign sent in the SIP messaging.
