> needs to be some servers available to facilitate the holepunching

Not needed. Other phones can help you determine connectable ports. You can create a phone-to-phone overlay and even puncture carrier-grade NATs. See our birthday paradox attack and initial dozen SIM cards measurements: https://arxiv.org/abs/2311.04658 Older university project, similar to Holepunch with more dev docs: github.com/Tribler/trustchain-superapp/