[lucaspiller]

I switched to a different SIP provider as they were cheaper, but my number was still held at the old SIP provider and couldn't be ported. I explained the story and asked if they could 'virtually' add that number to my account so outgoing calls would come from that number. They just switched on the feature to enable me to set the caller id to anything as it was easier for them.

[MichaelGG]

This is also a fun attack. Find a provider that does this. Request to port a target number (a bank or an escort service or whatever). Port will stall for a bit, in the mean time, the service provider activates your number internally, so their own dialers route to their "version" of the number.

Now you get all the calls from that provider to that number. Forward them to the actual destination (using an unrelated provider) and no one will notice for a while. Except, you get all the calls and media.

[WatchDog]

This assumes that the caller id has anything to do with how the calls are routed.

[MichaelGG]

Sure. Just sometimes, they accomplish that by adding the number into their internal inventory as if it already had ported.