Harry,
You came here for advice, and had some the best people there are anywhere in terms of NBU knowledge and experience reply to you.
I myself am one of the BackLine Engineers, so although I won't class myself as an expert, I do have quite a lot of experience.
Do NOT make changes to NetBackup, the way to fix this is for the customer to change the ports Oracle are using, there are no other options - so the choice they have is either change Oracle or leave the backups broken - up to them. I personally would noty tell them about client connect options so that they have no other choice (but see below for an additional reason).
Currently NBU will fall back to vnetd , this is true. This is not designed to be an alternative to PBX, it is left over from when it used to work that way. Then PBX was introduced as the 'only' contact, but vnetd will still work. I believe the plan is to remove the 'vnetd' option in the furture, menaing PBX will have to be used, when that happens, any systems configured the way your are considering will cease to work.
As Marianne/ Nicolai pointed out :
https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt
1556 is reserved for PBX, nothing else should be configured to use it. That alone is good enough reason to tell you're end customer to change.
Martin
