You can enable static outbound IP in your service settings.

Sorry. I meant INBOUND. I have mistakenly written outbound.

How can we get static inbound IP?

There isn't a way to get a static inbound IP. You'd need to use a custom domain or open a TCP proxy (depending on your use case).

Can we get a list of IP addresses that our service would use?

We can whitelist all of these IPs?

Railway doesn't offer static inbound IPs.

We have tried custome domain. it didn't work. the payment provider send requests to our service only with a static inbound IP.

the only thing left in your recommendation is spinning up a service on another platform.

We can move out to Digital Ocean in this case.

When an inbound IP address gets changed? Is it when we redeploy a service?

I'm not sure.

Can we get IP address like these ones in the link? There are not static.

https://www.cloudflare.com/ips/

You could try whitelisting these: