You can check the rates here:
For example, if you have a mobile subscription from Belgium Mobistar, you are currently in France and you want to call to Germany, this shows you how to calculate your price per minute:
There are 2 components:
1) Our call rate for calling to Germany: 0,0330 EUR/min;
2) Your roaming-in cost for France from Mobistar: 0,0847 EUR/min according to Mobistar’s website.
Total cost: 0,0330 EUR/min + 0,0847 EUR/min = 0,1177 EUR/min.
In comparison, if you wouldn't be using CT, you would be paying roaming out, which is always more expensive. In this example, that would cost you 0,2299 EUR/min according to: https://www.mobistar.be/nl/opties-en-diensten/internationaal/bellen-en-surfen-het-buitenland