The Starband service uses older technology that does not support voice well. Starband never intended for their service to support VoIP.
Clive is correct that you have the “jitters.” Terrible jitters in fact. You also have a case of the ‘longs’ as your long 250 ms packet lengths allow voice to be sampled only 4 times per second. You won’t get acceptable quality with that sampling rate.
I went into a little more detail about the technical reasons for this in a previous post for a user who has DirecWay…. same problem, same issue. No workaround but to upgrade to an enterprise class solution, which will cost more.