Try switching to a low-bandwidth VoIP codec like G.729 or even Opus (in low bitrate mode), if available in Zoiper. Additionally, see if you can enable jitter buffer settings in the app. This sounds like Zoiper may be using a codec that isn't ideal for satellite internet, which tends to have high latency and jitter. WhatsApp and Messenger use adaptive codecs made for bad connections VoIP apps like Zoiper often need a bit more manual tuning.