Model and Assumptions

This rule set is designed to track free credit usage for Tesco Mobile
UK customers topping up a regular monthly amount.

It assumes that no call or text bundles are in force, although it
could easily be amended to exclude SMSs where the 5000 (effectively
unlimited) text bundle is taken.

A monthly data usage indicator is included in anticipation of the
launch of 500MB/1GB monthly data bundles. This supports the data
bundle starting on a different day of the month from the free credit
allowance. It is assumed that no data charges are taken from free
credit (i.e. one of the new bundles or the current 7 day unlimited
'GPRS' bundle is purchased where mobile data is enabled) since this
would likely exhaust the free credit very quickly (at £4/MB or even
the promised new 60p/MB rate).


Setup

Ensure the rule set is installed on the latest version of Call Meter
3G (v3.0rc10 or later)

The monthly free credit allowance should be set by editing the "Free
Credit Usage" plan as 20, 30 or 40 under the Limit setting.

The days of the month on which free credit is awarded and the data
bundle starts should be set under Bill Period and Data Period
respectively.

In order for the model to work correctly, the International Prefix
should not be set and Strip Leading Zeros should be off. Both of these
are found in settings under Behaviour.

For those who signed up before 28th February 2011 and have Favourite
Numbers set up, these should be reflected in the Favourite Number
group (under Number groups). If you don't have or use favourite
numbers you can remove the relevant rules and plans relating to
favourite numbers.


Accuracy

The model is typically very accurate but may end up a few pence out
over a typical billing period due to the following:

* Call lengths measured by the network are rounded up to the next
second for billing purposes. This model uses the call length recorded
by the phone (to an accuracy of 1s), which mostly results in the right
answer but can occasionally under-calculate;

* Call charges are rounded up to the next 1p. This is accurately
modelled for favourite number and voicemail calls (12p and 15p/min
respectively), but Call Meter 3G cannot currently accurately model
other calls charged at 25p/min as each 1p increment does not equate to
a whole number of seconds (logged as issue 456). The 4s billing
increment currently set for these calls is designed to get as close as
possible to the right answer when averaged over a number of calls.