If the lack of response hasn't informed you, its not generally possible. With your software rev, you'd need the seperate Inter-Tel SIP Server platform tied into your Axxess system in order to implement in a generic SIP endpoint client device like the Linksys SPA. The normal Axxess VOIP setup doesn't usually have the SIP server installed.

Most PBX vendors find SIP too limiting, so they use their own proprietary VOIP protocols to do the signalling required.

I believe the newer Inter-Tel boxes (ie. 5000 series?) may have native SIP endpoint in them, but the Axxess did not, and your best bet is to find another IP SLA on the used market.