stupid solution... this working !
private string getIncomingNumber()
{
StreamReader sr = new StreamReader(File.Open("pjsip.log", FileMode.Open, FileAccess.Read, FileShare.ReadWrite));
string gg = sr.ReadToEnd();
sr.Close();
string[] matches;
matches = Regex.Matches(gg, "From: \"(.*?)\"", RegexOptions.IgnoreCase).Cast<Match>().Select(m => m.Groups[1].Value).ToArray();
return matches[matches.Length - 1];
}