Hello Mithun,
Your code may look like this
// create the node instance for activities with data
var nodeIns = this.Cust_Act_send.Create(noderef);
// TextCollection myNode is empty as this is newly created
// copy from PhoneActivity
foreach( var originalText in Phactivity.TextCollection) {
// new TextCollection, link to BOCustomActivity is set via path "this...."
var newText = this.Cust_Act_send.myNode.Create();
// copy values
newText.TypeCode.content = originalText.TypeCode.content;
newText.LanguageCode = originalText.LanguageCode;
// do not copy empty text
if (originalText.TextContent.IsSet()) {
newText.TextContent.Create();
newText.TextContent.Text.content = originalText.TextContent.Text.content;
}
}
Maybe the biggest difficulty is that a Dependent Object "knows" from the creation path to which instance of which BO it belongs to. 
Therefore there is not direct assignment to the "myNode" .
HTH,
Horst