If you are using MSAccess 2010 or later then you can use a
Data Macro[
^]
If you are using a version prior to that (likely with VB6) and are unable to upgrade then you will have to find another way.
However, I don't think you are going to be able to interact with a trigger with an external program in VB.NET. At best you will be able to put the SMS details on a "queue" (table) and have the VB.NET program monitor that at set intervals