Well, until today (and I don't know why) I had a concept in my head, that WSDL operation names need to be unique.
I'm really glad that guys from a big local mobile operator helped me clear things up (actually, they helped me lead myself into a no-exit alley). It happened today, during my web services day, a part of a bigger .NET training course.
I reread the spec. It says:
Since operation names are not required to be unique (for example, in the case of overloading of method names), the name attribute in the operation binding element might not be enough to uniquely identify an operation. In that case, the correct operation should be identified by providing the name attributes of the corresponding wsdl:input and wsdl:output elements.
Polymorphic methods are my friends again. I was mislead about the WSDL serialization details in that scenario.