deployment of ISR 4321 Router firmware

[monasir]

Hi everyone,

For a customer we are going to implement 2 cubes ISR 4321 for voice functionality.

Does anyone know which firmware the best i can use?

I'm trying to download the firmware , but little lost which one i need

[balaji.bandi]

 IOS-XE Releases 15.4.X  and above is good to go, but read the release notes and Refer below Document for the needs :

https://www.cisco.com/c/en/us/support/docs/routers/4000-series-integrated-services-routers/118792-config-isr-00.html#anc2