AFAIK it is true. It definitely was when O2 were BT cellnet. T-mobile is the
most expensive network to call hence this is where they get some of their
cash from obviously.

It's more or less true, sometimes.

When you all from a BT phone, you are charged according to which
network issued the number, not the network it is on now.

Calling an O2 mobile number at weekends from BT is 2p per minute
(minimum 5p, of course). No matter which network that number is now
on.

It's not a lot cheaper than calling other networks at other times,
though. Details form the BT web site, or from the Magsys web site.

http://www.serviceview.bt.com/list/current/docs/Call_Charges.boo/00161.htm

Daytime / Evening / Weekend
pence per minute inc VAT

fm1 (O2) 16.81  13.16  2.35
fm3 (TM) 19.33  15.16  3.65
fm4 (O ) 17.63  14.10  5.53
fm5 (V ) 18.92   7.29  3.98

So;

O2 is 13% cheaper to call than TM daytime.
O2 is 58% cheaper to call than Orange at weekends.
Voda is 50% cheaper to call than everyone else in the evening.

That last one is very strange - maybe someone at Voda finger-fumbled when
emailing evening termination costs to BT?

Yup, it sure is. Significantly so at weekends.

Vince