Why, after using Background(), something like sending a call gets into the conversation?

Question

Asterisk

The situation is as follows:
The city comes to the PBX si2000 and is sent to Asterisk by sip.
On Asterisk, the simplest voice menu, where by the number 1 everything returns back to si2000 to the local number.
That is:
city => {
Dial(SIP/si2000/local, 180, t); hangup();
};
Everything is great. But there is no menu :)
Adding a menu.
urban => {
Background(menu);
};
1 => {
Dial(SIP/si2000/local, 180, t); hangup();
};
Everything seems to work, but, after connecting, the _caller_ hears in the handset, along with the conversation, something similar to making a call, moreover, some kind of clumsy one. The one who is being called is almost inaudible, a light, rather noise, on the verge of audibility, while the other has such a beep that it is impossible to talk.
What could it be?
the menu is short, about 7 seconds. I played with Answer, WaitExtens, it does not affect in any way.

Answer 1

Background, most likely, had nothing to do with it.
Before the specified example, the dialplan also had a global selector of numbers of the form
ext => Dial (Local, ...),
which, it seems, generated an unnecessary call.
Replaced in the selector with goto - the problem disappeared