When we want to add a SIP trunk where the Authentication ID is the same as used on another SIP trunk, then we get the folloling error message: "Authentication ID provided is already used for another external line". Adding the same SIP trunk credentials twice is very usefull when you want to select different "Outbound Caller ID" by selecting specific SIP trunks within 3CX. The 3CX does not allow this, and we do not see any reason why the 3CX system should block this. With other VOIP systems it is also possible. We know that we also can define the Outbound Caller ID via the extension, but this is also not what we want. On an extension we want to use different Outbound Caller ID based on a dialed prefix. So e.g. when the number starts with a “1” then it takes trunk1 with Outbound Caller ID 1 when the number starts with a “2” then it takes trunk2 with Outbound Caller ID 2 Both trunks should use the same SIP trunk of the provider, and therefore we need to add the same credentials for both SIP trunks in 3CX.