Conceptually you are looking at a structure of drivers having devices having addresses. But all devices from all drivers are in just one big list numbered 0 to 'dwNumDevs' (what you got back from LineInitializeEx) .

So you don't yet know which devices are your avaya's. Usually the OS has several built-in devices taking up the first few slots. Your errors are probably from trying to access these. You first need to look through the device list to find the ones you want.

Use lineGetDevCaps https://learn.microsoft.com/en-us/windows/win32/api/tapi/nf-tapi-linegetdevcaps to ask details for each device in the list. You want to look at field like 'DeviceName' and 'ProviderInfo' to spot your extensions. Only then do you know which devices to use for lineOpen.

Please note that if you are doing this from C++ you wish to negotiate the 2.2 version not the 3.0 version (3.x use COM)