Putting a RJ 12 plug into a RJ 45 socket is sometimes can damage the pins. If it is using POE this could be the issue as pins 1 & 8 are generally the ones that bend back in the RJ45 socket.

Sounds like it is definitely a power issue.

POE uses pins 4&5 for positive and your customer has had DC line voltage across there and probably ring voltage. So the phone could be damaged but you would expect the phone not to work at all if that were the case.

Why do yo need to forward a port a the remote IP phone site?

Either way you need to get the thing back and look at it yourself.