How do I redirect SIP server VOIP ports (such as 8025) to a subdomain such as sip.mysitename.com, instead of the SIP server IP address?

After searching, I haven’t found an answer to this, but I may not be using the correct terminology. Corrections welcomed.

We have a VOIP server in the cloud, with an IP address, and a custom port. When we enter this into a softphone, it works fine. But we’d rather not have to remember the IP address, and instead use a subdomain such as sip.mysitename.com for an easier time memorizing it.

But when we set up sip.mysitename.com as a redirect to the VOIP server IP (with or without :8025 at the end), the softphones fail to register.

How can we do this?