I wanna hardcode id of hs_call_disposition.
Does the internal values for the call outcome change? Or i can use these ids from documentation
Yes, these values will not change and you can and must use them for the hs_call_disposition.
What about custom outcomes ? Is there any way to get the matching values somewhere ?
Unfortunately, my answer is not going to be very satisfying 
Typically, I would recommend querying the properties endpoint for that property
eg: https://api.hubapi.com/crm/v3/properties/calls/hs_call_disposition
this will return an options array. If you do that, you will find that the array is empty…
I have already reported this issue and it is on the to do list of the product team. No idea on timeline, however.
My hacky not very elegant workaround presently is set the custom outcome in the UI for a record. Then, querying the get a call endpoint and add hs_call_disposition as a property to be returned.
eg: https://api.hubapi.com/crm/v3/objects/calls/21532199002?properties=hs_call_disposition
This will return that property value like this
"properties": {
"hs_call_disposition": "9e70ad67-6843-4f39-a992-fc8bf301a56c",
"hs_createdate": "2022-05-05T14:56:34.728Z",
"hs_lastmodifieddate": "2022-05-05T14:56:35.204Z",
"hs_object_id": "21532199002"
},
I know it is not ideal, but hopefully that will help until the correct method is added.
I’m having the same problem… 2 years later 
, @dennisedson , any idea if this can be fixed? I don’t have a call to query to get the specific property, but instead need a full list of the hs_call_disposition properties available.
Here is my issue: https://community.hubspot.com/t5/APIs-Integrations/CRM-gt-Properties-gt-Core-API-gt-Get-quot-hs-call-disposition/m-p/1003791#M74912