The actual test is the same as an end-to-end test for SMS apps found in the “chapter” on Testing Messaging Apps (see the TOC). Essentially, it requires the tester to use a test handset and sim to send and receive SMSs according to the functional test cases for the app.
When performing in-market testing for SMS one thing to note is the traffic pattern for that particular country or operator. This means that at different times of the day, the network latency differs. For most operators, the peak traffic periods are (a) in the morning before start of work, (b) around lunch and (c) in the late evenings.
For purposes of getting a realistic profile of how the app behaves with real traffic, a good idea would be to conduct a test every hour (or key times of the day) over key days of the week. This is especially so if the app has session timeouts assumptions.