The most likely reason is that your Westminster City Council Business Master Administrator has not added your details (mobile number and/or vehicle registration) to the Corporate account correctly or the pre-paid account does not have sufficient funds to complete your transaction. You should contact your Master Administrator or employer to query this in the first instance.
If your mobile number is withheld or withdrawn then you will also not be offered Westminster City Council Business as a payment option. You should amend your caller identity settings on your mobile device and try again.
Similarly, it is only possible to call from a different mobile number other than the number registered by your Corporate Master Administrator if they have linked your Corporate permissions to a specific vehicle.