What trunk do you use to dial out? For SIP trunk it can be kind of tricky and it depends on how the SIP provider how they handle the calls with the hidden caller ID. We have one that requires a special prefix to be dialed e.g. instead of 9+DDI number you dial 9+code+DDI number. Another require a special DDI that the calls are going from. Another requires "Anonymous <sip:anonymous@anonymous.invalid>" in From part of the SIP INVITE message.