for 的問題

The sentence compares two things.

(1) the level of complaints "about" BT's landline telephone and broadband services
(2) the level of complaints "for" BT Vision ("about" television services)

In the second part, the object of preposition (describing "complaints") is the company name, so we normally use such preposition as "against" and "for".
~ complaints against (somebody)
~ complaints for (somebody)

For the special uses of for in phrasal verbs at the front entry ,actually it should be than (that) for BT.
for as a preposition=concerning for
=intended for,
=occasion for,
It should be higher than (that) for BT.