I notice that Lycamobile France's web site now says "Facturation par Mo" and "Billing is per MB". Although the use of the word "per" literally indicates that the prices are quoted per megabyte, the word is often misused by operators to indicate the billing increments. For example if a petrol station says that prices are "per litre", this does not mean that the charge will be rounded up to the next whole litre. If the latter is the intended meaning, it means that unless you manage to maintain long continuous data sessions (and most phones do not even indicate whether you do or not), you will be overcharged for data very often.

it is very hard to detect what is exactly a session for MNO/MVNO.... reading a forum and see 2 google adsense ads is considered as é or 3 sessions ?

i think you try once but not twice after listening the "321 / suivi conso" ^^