The RTD imposed by a satellite depends on a number of variables. Latitude + longitude of earth station, angular position of satellite, along with the radius of the orbit and the speed of light which are fixed, gives you the RTD, which will be approx 500ms. Other delays in VoIP are introduced in encoding, decoding, jitter buffers, router delay etc. The trick is to keep these to an absolute minimum since the satellite delay can not be reduced without breaking laws of physics. Voice quality is measured in MOS which takes into account various factors including delay. I don’t know of any standard limit for voice delay, but it starts to get noticeable if you go more than 300ms one way delay, some international carriers use this as a limiting figure. Remember international phonecalls over traditional TDM have always used satellites….