French salaries for senior developers are less than entry level developers in the US.
I have bootcamp students with 6 months of software development experience making more than top developers in France, lol. The few extra Euros you get for being on call hardly seems worth it.
There's a reason French workers are almost always rioting. Low pay and very few opportunities.
I don't know about France but in my country that's also factored in the law and hiring offers except for jobs that are on managerial level (them the law says you don't have clock in but you also pretty much don't get to clock out and you mostly get paid well for that), which doesn't apply for developers and that means you get paid less if there is no "on call". Basically your base pay is the same on company A and company B (assuming they are truly equal), but since company B requires "on call" that is extra work that gets compensated based on your hourly rate or double that if it is during night time, in the same way for the same job in the same company who ever works night shifts get paid twice as much as the daily shift (before taxes).