Lower Your Phone Bill! (Yeah, Right...)

This morning while lounging indolently on the couch finishing my coffee (or was I working on the last details of the lecture I was preparing to give to Štěpanka’s class on European Immigration? We’ll say it was the latter), I was roused by a loud knock on the door.
As a rule, no one knocks on my door except, once in a great while, my letter carrier. After all, it is an apartment building and visitors have two locked doors to get to before they reach my doorstep. So I thought it must be a package, although I was not expecting one.
No! It was a nicely dressed woman who wanted to discuss my telephone service!
Since I hadn’t yet paid the latest bill and have a few questions about it, I thought perhaps she was here in regard to that. After all, she was clear that she was here to discuss my land-line.
Still, I was a little skeptical. I was not sure whether she was actually representing Český Telecom. Nothing in her hand seemed to bear its name or emblem. Although I understood a good deal of what she was saying—she emphatically wanted to reduce my phone bill—I don’t really like to conduct complicated and potentially financial discussions in Czech. Thus, I put on a bit of a dumb foreigner act and said that I wasn’t sure I understood since I only speak a little Czech. Perhaps unwisely, I had already stated that the phone wasn’t in my name, that Sandra is not my daughter, and that I don’t know her birthdate. (People don’t usually expect me to have adult children, but I was dressed in a more mature manner than usual in preparation for lecturing to a university class. Once in awhile I apparently look my age.)
My visitor was not fazed by my claim not to speak much Czech, but informed me that she would speak slowly. I would not say that she spoke exceptionally slowly, but she did enunciate well.
After much more about phone service than I could possibly absorb, my visitor seemed to think it was time to start writing up an order form. By this time, I was quite dubious that she had any relationship with Český Telecom; presumably she was in the employ of a competitor, although I had thought Český Telecom had no competition on land lines.
While it had not done any good to say that the phone wasn’t in my name or that I didn’t speak much Czech, when I finally stated “Ale, nesmím změnit servis!” (But I’m not allowed to change the service)—even though it would have been more correct to say “služba” than “servis”—she suddenly got the picture, crossed out her form decisively, thanked me, and said goodbye.
Perhaps “nesmím změnit služba” has the same convenient effect here as “Please take me off your list” does for American telephone solicitors. I will have to remember this. (And find out whether the verb smět takes the nominative, accusative, or some other case.)