I always used to ring their desk at the port office… using the number on the ticket…
Numbers for the post offices are listed on SayNoTo0870
Like a lot of companies,for some reason since Covid started they have completely lost the plot with customer services.
Have had same problem, emailed them a number of times no reply.
Cannot phone them either
Looks like P&O are in worse bother than BF
Looks like the advice to call the French customer services is good - I wanted to change a seat booking on my up-coming crossing, after 10x attempts to sall the UK CS, with the line continuously engaged I got straight through on the French number.
The operator there was more than happy to help and very patient with a poor mobile connection at my end (it’s never good in the building). Sadly availability (or lack thereof) prevented making the change that I wanted but if you are in the UK and can’t get through on the 0330 number and don’t mind making a call to France it might well be worth it.