The Google Pixel 9a technically supports VoLTE, and Sunrise offers VoLTE telephony on the Swiss network. However, there is no guarantee that the Pixel 9a will automatically use VoLTE on Sunrise, as Sunrise (like other providers) controls device VoLTE compatibility by network profile. You cannot safely assume that VoLTE will work on Sunrise without explicit confirmation from Sunrise or a successful field test on the network.