You have tagged onto a Thread that has been dormant for 3 years and which seems to involve a Messenger library that you are not using.
serial input basics has simple and reliable functions to receive data. There is also a parse example.
There is another current Thread about the Sabertooth Library and SoftwareSerial. It may be that they are incompatible. It would make more sense to read or maybe even join in with that Thread
...R