Hey @amy6586 
The response is an array so you would need to use jsonData[0].operatingCarrierCode to reference the object within it.
As for assertions, Postman uses chai.js for this and more info on what could be used can be found here:
There are some test examples here on our learning center that also might help:
This is also a similar issue covered by @vdespa in one of his YouTube videos:
