300000 rubles for 1 gigabyte and 254 rubles per minute of conversation can be taken from you by mobile operators, despite all your unlimited Internet and the included options for cheap roaming, if you happen to find yourself in some points in Russia.

In some places of Russia there are zones where there is no connection of your operator. In these places another operator’s connection can be automatically provided. This is called “National roaming” (not to be confused with the operator’s intranet roaming in Russia).
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MTS has such zones in the Yamalo-Nenets and Khanty-Mansi Autonomous Districts, the Perm Territory, the Volgograd and Penza Regions. Prices there are (source:
static.mts.ru/upload/images/files/MTSNationalRoamingInstruction.pdf ):

Yes, yes, 8.6 rubles for 40 kilobytes, respectively 215 rubles for 1 megabyte. If your iPhone unexpectedly wants to upgrade, being in one of these zones and downloads 50 megabytes, it will cost you almost eleven thousand rubles. No discounts and options are valid.
Beeline has such a zone in Birobidzhan, I have not found a complete list of zones. National roaming tariffs there are:

As you can see, here a megabyte costs already 300 rubles, and a call to an international number is 254 rubles.
The Megafon press service assured me that Megafon does not have national roaming. There is free emergency internetwork roaming (AMR), which can be enabled in emergency situations.
There is only one radical way to deal with unexpected expenses in national roaming: disabling the operator’s auto-selection in the phone menu, however, many phones cannot choose to switch from 2G to 3G because they are different networks, although they belong to the same operator.
At MTS, you can activate the “GPRS-roaming ban” option and disable the “international and national roaming” service. However, international roaming will also turn off. Beeline has no options to combat national roaming.
And now I will tell you how I found out about this. In each of the working vehicles, we have GPS trackers with MTS sim cards and MTS Connect-4 tariff without options. In this tariff, 1 MB costs 1.5 rubles all over Russia with tariffs of 1 kb. The tracker sends units of bytes, so all charges are 0.15 kopecks for each session of the tracker.
Last week, when the car was in the Volgograd region, data suddenly stopped flowing. It was found that the account of the sim card tracker minus nine rubles. I was surprised and replenished the account for 100 rubles (usually this is enough for more than a year). An hour later, the data again stopped flowing. And the account turned out to be minus ninety rubles. I requested the details and saw this:

The cost of a kilobyte has grown more than 6000 times!
After talking with the MTS press service, money was returned to the tracker account.
Of course, there are vanishingly few places where there is no operator’s network and automatic connection to another operator, but God forbid you be in one of them.