What you forget is when an account is slammed this is classed as a new contract between the householder and the gaining ISP the old ISP has no say or right to contact them.
Lets say all conpanies were forced to comply to OFCOM MAC code then perhaps the losing ISP could do something as it is such as sky, talktalk do not need MAC codes apparently since they move the line of BT network. Oh yes this would void the contract with the ISP as the home no longer has a working BT line.
Also as I say the person living in the property at the time must have made some contact with plusnet to give out the information required to slam the line. Once you do this then you are left open to misinterpretation of a new contract with the gaining ISP. A little suprised about the lack of MAC code then again he might have also given one of them.
Since the OP isn't the person who was living in the premises at the time he doesn't know how much information was given out.
There is so much myth in this the real world is not as kind. Contracts need to be read sadly 90% do not even read them but to give out enough info to allow slamming doesn't leave the blame with the losing ISP you really should try this in court if you feel so sure.
My son returned home Thursday evening to find a note from BT/PlusNet that his phone, new phone number and internet connection had been moved to PlusNet by someone who previosly lived there and now resides in North Wales.
This can imply more than one thing returned home from work, holiday, working overseas, the OP didn't give enough information to to enlighten us.
the losing ISP will have problems talking to plusnet since the contract between plusnet and the customer does not include them. I am sure they do not want to pay for a new line plus new activation for something that might have been done by a person who moved out. Before you say anything this is often a way to get even if you fall out with someone you were sharing accommodation with so worth also asking had they had problems sharing the home or was this other person renting and had to leave as the OP's son was returning.
If the ISP didn't instigate any move from them and a customer moves this tends to imply the customer has broken the contract with the ISP. The ISP didn't break any contract the home now has a new number, so the active line wasn't migrated over therefore a cease was placed perhaps for unpaid bills the other person doesn't want to admit to.
I can come up with lots of ways this wouldn't be the losing ISP fault mainly because I see both sides of this problem. Since neither the OP or his son were the person who dealt with plusnet they do not know what was said or agreed to only the person in Wales knows.
Theories as much as you want each scenario is different but once the phone changed number the contract was broken by someone in the property.
1. Why the new number?
Answers possibly the phone was with BT but broadband with an LLU, phone ceased for some reason normally none payment and openreach messed up and didn't cease the broadband.
2. So why the new number?
Only new numbers are given on restarts or new lines.
So we can see there was a problem somewhere but this wasn't with the ISP since you cannot get a new number unless the old number was ceased.
All contracts for broadband say you have to have a working BT line new number means there was at the time no working BT line so contract broken by customer since the premises had no working BT line.
So the main question here is why the premises needed a new number perhaps the OP could say if it was due to cancellation from the pstn supplier?