For outbound dialing u can go for an outbound dialer solution ( Hardware Based ) and u will not require any EPABX , AVAYA etc. since u are not having any inbound calls.
Go for DAVOX (now CONCERTO) or MOSAIC .
But these solutions are a bit expensive as such u can look for a local solution in your place.
You will find many such non- branded solutions available in the local market. Just evaluate them.
I dont know where u r based .
Do let me know ?
I am in INDIA and we have many such home grown solutions readily available here.
Further , if u have a customer database and are into some sort of CRM then both can be integrated also and u can have a SCREEN POP-UP too once the call is answered by your customer.
U can do without an AVAYA phone or the AVAYA switch .Just go for a non-properietary phone to save costs.
In case u require any further info. feel free to write to me.
Regards and Best wishes