On a number of occasions, I've noticed that Orange have billed me or my wife
for outgoing roaming calls, not by the second, but rounded up to the next 4
seconds, 30 seconds or 60 seconds etc.  This happens when Orange say on
their web site for the network concerned "Our partner network [network name]
in [country] charges calls per second".

Before Orange's standardised roaming charges were introduced in 2001, the
"partner network" would determine all outgoing call charges, and they would
simply bill Orange for the charge, which would appear in GBP on the Orange
customer's bill.  Now that Orange sets the per-minute rate but the partner
network still sets the billing increment, who calculates the charge for a
particular call?  Orange or the partner network?

I would guess that it is in fact Orange that calculates the call charge,
since Orange presumes to know the billing increment of every partner
network.  Furthermore, the partner network would not necessarily know
Orange's per-minute rates, nor would it operate in GBP, so it seems unlikely
that the partner network would have any role in the calculation of the call
charge that appears on an Orange customer's bill.  Perhaps though, the
partner network passes two call durations to Orange - actual duration and
billed duration.  Does anyone know for sure how this works?  In other words,
who is to blame for these errors?