If you are saying you are using T-Mobile, they do throttle everything once you reach a certain usage depending on your plan. They slow all the way down to EDGE speed so maybe thats the issue?? Now if you are their 4G not LTE the latency on it is terrible, but other than that with the phone you are using you shouldn't be having any issues. I use G729 on S4's, S5's, Note2 & Note3's, HTC One's with no problems. The only phones I have had issues with are the Motorola phones where I have audio problems. I use AT&T, Sprint, and Verizon all with no issues using G729. I use Mediaphone and Zoiper for my softphone BTW.