The following list where the model numbers are mostly found on the following telephone.
Aastra/Mitel: Most model numbers are in the top right of the phone on the front.
Avaya/Merlin : Most model numbers will be found under the handset
AT&T/Lucent/ Avaya Partner MLS : Top Right Corner
Cisco IP : Usually on top, above the display screen.
NEC : On top, above the display screen
Nortel/Meridian : Underneath the phone, towards the bottom
Panasonic : Usually in the top right corner of the phone
Polycom: Usually on the bottom of the phone, most model numbers start with VVX (victor victor xray, not w) or Soundpoint IP
Shoretel: Most model numbers are on the front of the phone, bottom right.
Toshiba : Underneath the phone
Vodavi: Upper right corner
Yealink: Under the phone on the sticker. All Yealink phone models start with SIP
Zultys: Front of the phone, top right. All Zultys models start with ZIP.