hm, lplus is a German company, seated in Bremen, and according to the LBA (German CAA)
list of accredited language schools, it is not listed.
As a level 4 examiner I was taught certain methods for doing the examination and I would NEVER do it over the phone! There are ways for proving that English is your mother language and sending that proof to the authorities to check it (my American passport, plus I had a conversation with the LBA), but it otherwise needs a proper evaluation.
I'm sure there are different ways of doing it in different countries. But to have your level 6 added to your UK JAR-FCL it would, I think, have to be an examiner/school approved by the UK CAA.