Unless something has drastically changed with 3rd party SIP phones, you'll need an Avaya phone to be able to use the majority of programmable button features such as user buttons, group buttons, etc. You're pretty much stuck with what can be handled via shortcode, outside of features like transfer, conference and forward which can be handled at the phone level.
Someone may prove me wrong. About 7 years back I had a customer who wanted to reuse Polycom phones. In the end, the extra price for the 3rd party licenses and the lack of features drove them to abandon the Poly phones and get native Avaya phones.