Seems like that should work to me. We have a site with Avaya VoIP phones using a 3750 switch, and a similar configuration works correctly for us. Double check just to make sure that your Voice VLAN exists on your switch (show vlan). Are you seeing mac-addresses from the Nortel phones on the data VLAN?
Do you have Layer 3 routing enabled on the 3550? If so, try disabling that unless you need it.
As another test, on your 3550, turn on "term mon" and turn on "debug ip dhcp server packet". Then reboot one of your phones and provide the debug output. That will show what the switch is seeing at least. You may have to add "ip helper-address x.x.x.x" to point your VLAN interfaces to your DHCP server depending on your configuration.
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