Try doing

  $txt->attention;
  sleep(2);
  $txt->hangup;
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and see if that works. There is a call to attention() built into hangup(), but I looked at the code and I don't see an effective delay after attention() sends '+++' to the modem. There is just a comment about waiting "200 milliseconds". (If this code change gets it working, pare down the 2 secondsin sleep(). It's that big just for a test.)