Yes, you can use Avaya SIP phones on a Cisco system and vice versa.

Why? As you may be aware SIP is an open standard and is being adopted.

SIP is dumb.

Avaya phones work with SIP because of Avaya PROPRIETARY extensions.

Cisco works for the same reason and the extensions are added for features.

A basic SIP phone no mater who provides it is basically just a dumb lobby phone.

The entensions are the meat of the phone when using SIP.

Contrasted with SCCP=Skinny, do keep in mind that Cisco was an early adopter and they had to do something to make it work.

CDP-Cisco Discovery Protocol hands out VLAN info to a phone amongst other things.

LLDP- is the world trying to catch us to what Cisco has already been doing for years.
I read on here where this guys bashed CISCO for being a data company and shouldnt be doing VoIP.

Fear and ignorance can make us say dumb things.
Fear of the unknown perhaps is a better description.

Im a Nortel and Avaya guy and have been for 25 years. Only this year have I spent with my head in a Cisco book every single month of this year.

I dont live in the Matrix and perhaps I have drank the Cisco kool aid.

I came in to 2010 believing Nortel CS1000 5.5 is the best PBX in the world.

I still believe that today.

I was once a Toshiba certified engineer and a man told me that I would like Nortel more eventually. I doubted his words but they came to pass like he said.

Cisco is a whole new world for me having obtained certification on the Dimension 2000 and Stromberg Carlson MDM100 with X-Y switches.

I embrace it for what it is...Darn good Kool-aid.

CCNA
CCNA Voice
MCSE W2K3
Security +
CS1000 5.5
Toshiba
Comdial
Avaya
Mitel
Norstar