You can check how many people are using various numbers here
As you can see for 0845 the number of callers is in single figures. From what I remember the 0845 number didn't fail every single time, but did fail 90%+ of the time so couldn't be relied upon.
You can configure your tivo to use other numbers, although unless you have access to a bash prompt using a serial connection you will need to put the tivo drive into a PC to do it. The details are here
You could substitute an ISP phone number, account, and password of your choice.
There is a downside however as normal ISP access numbers don't have the 1 hour hangup by the server. If the tivo doesn't hang up you can quickly run up a very large phone bill.