Now, I don't know Swiss or German Law - but What you've described sounds like Standby fee/Standby Duty.
That is, if you are expected to drop everything and come into work, then you are paid either a set fee or percentage of your hourly rate.
For example - If I'm on-call with a company and I'm required to be within a certain distance of the Office, that limits my freedom to enjoy my time off, therefore I'm to be compensated for it. In NZ, I had to be within an hour of the Datacentre, and I was paid a flat-rate just for being on-call and then claimed any additional hours I worked.
Now - reading some German Law - there are two concepts - On-Call Duty (where there is no legal requirement to be paid) and Stand-by Duty (where there is).
The differentiation is:
For the correct classification, it is important whether the employee can actually freely choose his or her place of residence. The European Court of Justice (ECJ) recently ruled that a period of time described as "on-call duty" must actually be classified as working time (and thus does not constitute on-call duty, but rather standby duty) if the employee can freely determine his or her location "on paper", but is nevertheless significantly impaired in the way he or she spends his or her free time (for example, due to very short response times).
In short, based on my understanding, if your Employer is placing a restriction on you "Don't go up the Alps for a Ski weekend, in case I need you to come in" - then they are considering you on Stand-by and need to pay-up.
Obviously though - Check with a local Employment Lawyer or advice bureau.