Question: I'm looking for a game backend service that supports cross-platform multiplayer development for various gaming platforms.

If you need a game backend service that lets you build cross-platform multiplayer games, Photon Engine is a good option. It offers advanced state synchronization, high-performance deterministic performance and support for Android, iOS, .NET, Mac OS, Unity 3D, Windows and Unreal Engine. Photon Engine also offers voice and chat support, a real-time API for matchmaking and room management, and cloud hosting with low-latency servers in many regions. It also offers tiered pricing with a free plan and upgrade options, so it can be used by developers and studios of all sizes.

Another good option is Azure PlayFab, a full LiveOps backend service that spans the entire life cycle of live games across clouds, devices and game engines. It offers multiplayer services, analytics, live operations, party networking, matchmaking and cross-network identity, as well as strong security and compliance options. PlayFab's pricing is flexible, starting with a free account and including a $200 credit for the first 30 days, then a pay-as-you-go pricing model.

If you want an Azure-based service, Microsoft Azure PlayFab is a collection of services geared for game developers. It offers multiplayer services, live operations, real-time analytics and compliance with Xbox, PlayStation, Nintendo Switch and Facebook. Azure PlayFab offers a free development mode and pay-as-you-go pricing, so it's good for developers of all sizes.

Vivox is another good option, focusing on in-game voice chat software. It offers cross-play compatible chat, integrates with game engines like Unity and Unreal, and offers advanced features like 3D Positional Voice Chat, Text Chat, and full accessibility support. Vivox is designed to help build active gaming communities with real-time communication tools and can be integrated in less than two days.