Currently, Nextcloud does its .vcf export phone numbers like this example:

TEL;TYPE="WORK,VOICE";VALUE=UNKNOWN:+49123456789
which will not be recognized because of the quotes.

In form1.cs, you could change the line 1426 to

switch (type.Replace("\"", ""))

that would do the job, but maybe there is a better place for doing the replace, so I don't add a pull request.