Vodafone has made PAYG consistently more difficult and expensive over the last 10 years - the bottom line is that they don't seem to want PAYG customers anymore - unless you are willing to pay over the odds for everything. Plus, as you have found out, it's actually quite difficult to get information on the PAYG system.
At last recall, you paid a hefty price when a call was answered and thereafter a minute by minute charge - so you might be charged €3 to connect the call and then a charge-per-minute after that.
We were more or less forced to change to a contract a few years ago as PAYG had just got more expensive than a contract - for example, if you bought PAYG airtime or any "bundles", they would expire after 1 or 3 months and you would lose any balance left on the airtime or bundle.
Since we changed to a contract, the cheapest contract has escalated in price, possibly making PAYG worthwhile again. Who knows?