Is the free HBO max only on for watching on your phone?

As long as your Directv account has the same billing address as your wireless account, you get HBO on Directv too. I get HBO Max through my wireless plan too and it is added free on my Directv account. As noted, HBO Max is a streaming service, what you get on Directv are the HBO satellite channels. There is some content on the streaming service you can’t get from the satellite channels.

No hbo maxis a streaming service not available through directtv 

No 

When I log into my at&t account and go to the hbo max link and download the app it’s saying free trial and asking for my credit card information Is this correct?

Will it automatically come up as an available channel on the direct tv or do I need to call 

Download the app google the compatible devices for it. It doesn’t come over your directv receiver

When I log into my at&t account and go to the hbo max link and download the app it’s saying free trial and asking for my credit card information Is this correct?

No, that is not correct. You use your wireless account credentials  to access the app but you must first authorize your account as having HBO Max access.

Will it automatically come up as an available channel on the direct tv or do I need to call 

Once again, HBO Max is not a satellite channel. Once you have successfully created your HBO Max account, HBO should get added to your Directv account also. You will then have access to the HBO satellite channels.