Working out which mobile-phone plan offers consumers the best value is hard to do. A lot depends on what times users make calls, who they are calling and how many minutes they spend on the phone.

But one thing seems clear: the more times users press the green phone symbol on their handset and the longer they keep it glued to their ear, the more cost-efficient it will be to abandon the "top-ups" of pre-paid in favour of a bill.

Both Vodafone and O2 explain the fact that the Republic has the highest average-revenue-per-user (ARPU) figures in Europe by saying that Irish people spend longer talking on mobiles than our European neighbours.

ARPU figures for the pre-paid market have been consistently high in the State, despite the fact that an OECD study found that pre-paid prices were the third cheapest in the EU. This suggests that Irish pre-paid users do use their mobiles to talk and text a lot more than pre-paid users in other countries.

So why are the 70 per cent of Irish people who use pre-paid not switching to post-paid, when billed services - with their bundles of "free minutes" and lower per-minute charges - tend to be more economical for heavy users?

Are hundreds of thousands of mobile users paying over the odds simply to avoid the commitment of a monthly bill?

According to the Consumers' Association of Ireland (CAI), there are times when calls cost four times more on pre-paid than on post-paid phones.

The differences in call charges are offset to a degree by a monthly rental charge on post-paid contracts.

If consumers use their mobile phones for a certain amount of time each month, they will cross the line where this charge will be swallowed up by the fact that per-minute call costs on post-paid are lower.

"The likelihood is that - if a bill suits your lifestyle - you would possibly be better off paying by bill if you use your phone for more than 100 minutes a month," says a spokeswoman for Vodafone, which dismisses the CAI's "four times more" figure as selective.

"Obviously, there are people who would pay less if they paid by a bill, but they like to use pre-pay. It's just the way they prefer to do it," she adds.

Vodafone says 70 per cent, or 1.19 million, of its 1.7 million customers use pre-pay.

O2 has 903,000 pre-paying customers, representing 71 per cent of its customer base. It summarises the appeal of pre-pay as "no contract, no rental, customer is in control".

A spokeswoman for O2 says its salespeople ask customers a number of questions, including how often they use their phones, what kind of calls they are making, and at what times they are making them. The answers should help them decide whether they should switch from pre- to post-paid. After that, it is up to the customer to judge which option is best for them.

A spokesman for Meteor agrees that it is a personal choice, but adds that daytime users who use their mobile to run a very hectic social life - or for work purposes - should not pay as they go.

"If you are spending €30 or €40 a week and potentially spending 55 cents per minute on calls, then you shouldn't be on pre-pay," he says. "That's €120 or €160 per month on your mobile spend."

For €125, Meteor's highest-end post-paid plan, Meteor 1250, gives just short of 21 hours airtime a month. A person who makes mostly peak-time calls will be lucky to get more than four or five hours airtime on any pre-pay package for this sum.

Meteor, which has a market share of 4 per cent and does not yet have a fully national network, says it does not differentiate between pre-pay and post-paid prices to the same degree as Vodafone and O2. Its highest pre-pay rate is 55 cents compared to 65 cents at Vodafone and 63 cents at O2.

A study by Amárach Consulting, on behalf of telecoms regulator ComReg, shows that 55 per cent of people rank easily understood pricing as "very important" when choosing a mobile phone package.

The confusing myriad of promotions offered under post-paid plans makes them harder to compare. For example, many of the "free" minutes in Vodafone packages are restricted to same-network or landline calls. However, Vodafone Extra, which costs the same as O2's Ten + plan, includes 75 minutes of calls to any Irish number at any time.

O2 Ten + includes call value of €10. Even if the O2 customer only makes calls costing 15c per minute - i.e. to other O2 mobiles at any time or to Irish landlines during off-peak hours - they will only be able to make calls lasting a maximum of 67 minutes before they start paying extra.

When the O2 customer on this plan calls Vodafone or Meteor customers, they use their free call value at double the rate.

Expressing free calls in terms of their value rather than minutes like this makes it trickier to assess exactly how many and what type of conversations customers can squeeze in before they exceed their free-call eligibility.

Consumers will then have to assess whether or not O2's add-on options, one of which is included as standard, have more or less appeal than Vodafone's offer of 75 off-peak, same-network "call -a-friend" minutes and free voicemail.

It is worth noting that Meteor 75, which costs €5 less than O2 Ten + and Vodafone Extra, includes the same bundle of minutes to any Irish number as the Vodafone plan. Instead of same-network "call-a-friend" minutes, it adds on 75 free text messages.

Another difference between the operators that could dramatically affect the size of bills is how they charge per time of day.

Vodafone has a peak and an off-peak rate on its post-paid contracts, while Meteor simply offers a flat rate. O2 gives customers the choice between peak/off-peak and a flat rate, which, as well as staying the same at all times of the day, also remains the same regardless of whether the phone is being used to call Irish landlines, O2 Irish mobile numbers or other Irish networks.

This makes the flat rate attractive to O2 users who are calling a lot of landlines or other mobile networks during peak hours. It could also help users calculate call costs more automatically.

Some mobile-phone users will opt for pre-pay because they find it easier to keep track of their spending under a system where they must actively purchase and feed new top-up credit into their phone every so often.

Only 5 per cent of people told the ComReg survey that the ability to control their monthly spending was "not important".

A new system soon to be introduced by O2 should remove one difference between pre- and post-paid, making it easier for bill customers to assess how much debt they are racking up during their daily chats.

Vodafone and O2 already allow bill customers to track their total call costs since their last bill online. From next month, O2 will allow bill customers check how much they owe by entering a short code into their mobile handset.

The ComReg survey also shows that pre-paid options are preferred by nine out of every 10 people in the 15-24 age group and 97 per cent of students. It suggests that younger mobile-phone users, perhaps because they have less disposable income, are more price-conscious.

Indeed, you have to be 18 to pay by bill at Vodafone, while at O2, people under the age of 21 need a parent to act as guarantor. The Consumers' Association believes the higher call costs on pre-paid mean mobile-phone companies are exploiting their most vulnerable customers.

The CAI's presentation to a recent Oireachtas committee called for more consistent charges for the two types of customers. This would certainly simplify the dilemma of whether to pay as you go or pay later.

"Pre-paid customers should pay the same rates as post-paid," it said. "Pre-paid is popular with the young, students, part-time workers and the unemployed, who don't want to commit to a contract - surely, a lot of people in these categories deserve lower charges, not higher."