It may be of some help to you. We have once done the same thing in one of our project where we had to test USSD (Unstructured Supplementary Service Data) and SMS (Short Message Service) using the SMPP (Short Message Peer-to-Peer) protocols. Goal was to find a break-point where the CPU usage goes above 80% and Memory usage goes above 70%, for this we had to put load over the application with tones of SMS and USSD codes being fired.
We have used the tool SMPPSIM for this purpose, it provides the functionality of sending and receiving lots of SMS using the SMPP protocols. The only challenge was its configuration which took a lot time for me and my colleague to complete, for the USSD part we used both JMeter and SMPPSIM. Application has complete tree-node structure defined for purchasing products like buying a message pack which can be accessed via USSD code *111*12*3# (for example, this code is pointing to a product which can be purchased via USSD call). JMeter simulated load with 500 concurrent users running for Forever (Loop Count = Infinite) and then hitting the web interface of SMPPSIM which in turn was calling the USSD code *111*12*3# through different mobile numbers (test numbers) passed via CSV files to SMPPSIM and then buying 1000s of products simultaneously which increases load over the server to find the breakpoints. From the SMPPSIM and application logs we used to calculate that how many SMS and USSD products got success and how many got failed to deliver & check-out.
The irony of the solution was it took several days in its configuration (as everything was being configured over Linux servers and I didn't have knowledge of working over black screens with Linux commands), but it worked and served our purpose well. After that we presented report got the application performance tuned and released the project successfully. I will suggest you too to use the SMPPSIM but first need to do a little study about the same.