Apex domains cannot have CNAME records.
Question and response
Question
dig vpbx.avantel.ru. NS
Answer
vpbx.avantel.ru. 5 NS ns1.mostcom.ru.
vpbx.avantel.ru. 5 NS ns2.mostcom.ru.
Authority
The response has no authority records.
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0