@salma mhammad Thank you for reaching out to Microsoft Q&A, apologize for the delay in response.
Azure Communication Services is a great option for hosting a VOIP application. It provides a set of APIs and SDKs that allow you to add voice and video calling, chat, and SMS capabilities to your application. You can use the Azure Communication Services Calling SDK for Android to add VOIP calling to your Flutter app.
You can refer to this documentation on getting started with adding video calling using ACS to your android client app
The pricing for Azure Communication Services is based on usage, so you'll only pay for what you use. You can find more information about pricing of ACS here.
ACS also supports cross platform development using the UI libraries as mentioned here.
Feel free to reach back to me if you have any further questions on this.