teensy 3.5 interfacing with RockBlock 9603
The Teensy will be fine with 5 V on the inputs; that's what tolerant means. The main question is whether the 3.3 V output from the Teensy will be recognised as a logic HIGH by the RockBLOCK. The answer is in the datasheet:


  T/Rx High Minimum: 3.0 V


As long as the Teensy will drive the load to a voltage >3.0 V, it will be fine. 

However, given the cost of the RockBLOCK module, the addition of a level shifting circuit seems like a trivial addition to ensure the correct operation. The chip you linked isn't a level shifter for logic level signals, it's for the RS-232 interface. Just use a buffer to boost up the microcontroller outputs to the 5 V you need on the RockBLOCK side.