Each simultaneous call costs you ~100Kbps. An SMS is 140 bytes, and limited by desired SMS throughput. I use a T-Mobile femtocell with a SpaceX Dishy, but VSAT (64 Kbps up to 8 Mbps) should be fine for SMS only.

https://explorersweb.com/starlink-in-antarctica/

McMurdo could run their own BTS and provide SMS access only to mobile phones if the desire and funding was available. This would give you a legit mobile network registration.

http://openbts.org/

https://www.theverge.com/2022/10/6/23389641/ukama-open-sourc...