Avaya Index affinity switch h 232 ? & Asterisk question

Hi All, I work in a small managed office, we have 4 extensions from an Avaya index infinity switch system with Avaya 2030 phones, we have 1 DDI number, all we have access to is the phones in our office. Is it possible to have these 4 lines managed by (plugged into somehow) Asterisk (we only have access to the physical pones and cables etc in the office)? So then we can make outgoing call via VoIP and use the advanced PBX featured. Another thing to keep in mind we cannot have a permanent divert from the Avaya system, we get charged more for diverts.

I estimate if the above can be done we may be able to save 50% on our call charges.

If the above is possible then I can start the project and allocate budget and time.

Many thanks,