Many people seem that believe that mobile carriers act like non-profit organizations. They give them free calls, free texts, free gigabytes, and most importantly free (or 1€) smartphones that would otherwise cost hundreds of euros. And in exchange for those acts of pure kindness, they expect nothing but a tiny fee that is collected automatically every month without bothering the account owner.
On another end of the spectrum, some people believe that the only purpose of mobile carriers' existence is to steal as much money as possible from hard-working individuals. There are multiple mobile carriers to "choose" from, but they are all the same. And they just take from people's accounts as much money as they see fit, and we can only pray that they will show mercy and leave us some money for our survival.
In reality, mobile carriers are neither your friends nor your enemies. And if you follow a few general tips, you are much less likely to get ripped off in your relationship with them.
Know your needs and seek independent advice
It doesn't matter if you are looking for a mobile plan for a family of 5 with at least 100 GB per SIM card, or for the cheapest plan for calls only for your grandmother. You are probably not the only person who was in a similar situation, and someone somewhere probably compared offers from multiple carriers and was able to pick the best one. Just make sure that the advice you are acting on is independent (the post/article is not sponsored) and relatively recent (preferably not more than a few months old).
The smartphones (or any other devices) purchased from carriers are more expensive than those sold by electronic outlets
That 1000€ iPhone is not "free" with the 80€ plan. You are paying maybe 20€ for the plan, and 60€ installment for the device. And in most cases, if you were to get that phone from any electronic outlet that also offers installment plans, your total monthly payment (payment for the plan + installment payment) would be lower than the price for your plan with "free" smartphone.
That's not to mention that buying online and paying upfront would result in even cheaper total costs.
Get a plan with more GB than you expect to use
Unsurprisingly the carriers are making the most money on selling data allowance. And if you find yourself in need of additional data they will show you no mercy (it's not uncommon for them to decide that extra 5 or 10 GB of data costs much more than the total cost of your monthly plan). So if you decide to "save" on your data allowance, and then buy extra data "just a few times", it will probably turn out that you would be better off by getting the plan with a higher data allowance in the first place.
It doesn't pay to be loyal customer
It takes more effort to switch carriers than to remain with the current one. Therefore all carriers are more focused on obtaining new customers rather than retaining existing ones. So once your contract expires, it is likely that another carrier will have better offers for you.
And if you really are lazy and don't want to switch, when contacted by the carrier about extending your contract at least pretend that you are considering switching. You may get a better deal.
Don't sell your freedom every 2 years for a promise that they won't take advantage of you
I don't know what's worse about post-paid. The fact that the carrier can take as much money from your account as they see fit. Or that if something goes wrong on the carriers' side (and you want to terminate the relationship), you are expected to compensate them for that.
Did you use 1 MB of data near the border of another country? (including EU countries - for some reason that "free-roaming" thing was temporary). Too bad - we took 100€ from your account, if you don't like it you can switch in 2 years.
And the most traditional advantage of post-paid is no longer there. You can set automatic top-ups in most prepaid offers (so that you don't need to remember to top up your phone every month).
If you absolutely have to, get your "free" phone with pay-as-you-go contract
Pay-as-you-go contracts are more expensive than post-paid contracts. However, if you are in a situation where your phone broke, you don't have money to pay for your smartphone upfront, electronic outlets won't let you take installments and your only option to stay connected is to get a "free" phone from a mobile carrier, then your financial situation is probably not great.
And I agree that you should save every penny (or Eurocent, or Polish grosz, etc.) and the fact that you will be paying 3€ more for pay-as-you-go contact compared to a post-paid contract is problematic. However, if you do struggle financially, it is much better to spend 3€ more every month, but pay upfront and be safe. Rather than get a nasty surprise of 100€ disappearing from your account for no reason one day. It might be the carrier's mistake - they were supposed to charge someone else with your name that extra 100€. You complain and get the money back in 10 business days. From the carrier's perspective - no problem, business as usual. But from your perspective (if you didn't have that 100€ to pay for smartphone upfront in the first place) it means that surviving 10 business days without that 100€ (and that could be catastrophic).