Apex domains cannot have CNAME records.
Question and response
Question
dig swissvoiptel.ch. NS
Answer
swissvoiptel.ch. 5 NS vtns02.swissvoiptel.ch.
swissvoiptel.ch. 5 NS vtns01.swissvoiptel.ch.
swissvoiptel.ch. 5 NS vtns04.swissvoiptel.ch.
swissvoiptel.ch. 5 NS vtns03.swissvoiptel.ch.
Authority
The response has no authority records.
Additional
vtns01.swissvoiptel.ch. 5 A 194.6.184.54
vtns02.swissvoiptel.ch. 5 A 109.106.19.50
vtns03.swissvoiptel.ch. 5 A 194.6.184.55
vtns04.swissvoiptel.ch. 5 A 159.100.248.229
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0