toString method
A string representation of this object.
Some classes have a default textual representation,
often paired with a static parse function (like int.parse).
These classes will provide the textual representation as
their string representation.
Other classes have no meaningful textual representation
that a program will care about.
Such classes will typically override toString to provide
useful information when inspecting the object,
mainly for debugging or logging.
Implementation
@override
String toString() {
  String contact = '<';
  if (anonymous) {
    contact += temp_gruu ?? 'sip:anonymous@anonymous.invalid;transport=ws';
  } else {
    contact += pub_gruu ?? uri.toString();
  }
  if (outbound && (anonymous ? temp_gruu == null : pub_gruu == null)) {
    contact += ';ob';
  }
  contact += '>';
  return contact;
}