We are a new 3CX customer and trying to get it to work with our current SIP provider (Masergy). They are telling me the from header needs to be coming to them with a DID that is in their system. I have changed random outbound rules on my SIP, but not really sure which one, would give them what they need. I almost want to just buy out my contract and move to another provider. I tested 3CX with a few different providers that give you a free DID for a few hours, and had 0 issues... Hopefully someone cant point me in the right direction!