Welcome Robin.
As a TV controller it works fine ie The 2520 Remote happily controls the digital channels.
However when integrated with the Tivo system it is unreliable.
I take it that the Philips box using its own remote changes channels consistently but not changing channels with the Tivo remote or the Tivo itself making timed recordings?
One of the first Google entries for "Philips 2520 stb" is a thread in this forum with another new member unable to get reliable recording
http://www.tivocommunity.com/tivo-vb/showthread.php?p=4148642&&
from there...
Chumpers said:
The RedRat reports that the remote control for the Phillips unit sends a different code every time I use the same button
I've seen this when trying to program my Philips Pronto to control my Philips projector

What happens is the remote sends a slightly different code for each alternate button press so the STB (or whatever) can tell the difference between a single long key press (repeating the same code over and over) and two distinct presses (two different codes one after the other).
I don't think you will be able to get your Tivo to replicate this behaviour so that STB is unlikely to work with your Tivo.
On a more positive note STBs aren't very expensive and there are plenty of reliable ones that work well with Tivo. The subject comes up regularly on here - if you search you will find a number of recommendations - I would stick to the most recent and if you can buy from a place that will replace or refund you if your choice proves to be incompatible/unreliable e.g. Tesco, Asda etc.
I use a Wharfedale DV832B which is £25 at Argos and has not missed a channel change in almost a year of continuous use.
http://www.argos.co.uk/static/Product/partNumber/5320503.htm