Why don't you just make the Constructor of the Class take parameters:
public Contact(string firstname, string lastname, string phonenr)
{
this.FristName = firstname;
this.LastName = lastname;
this.PhoneNr = phonenr;
}
Of course, if you do that, you really don't need a separate 'CreateContact method.
If you expect that some contacts will not have a phone number, you can make that parameter optional:
public Contact(string firstname, string lastname, string phonenr = "")
{
this.FristName = firstname;
this.LastName = lastname;
this.PhoneNr = phonenr;
}