While not an expert on QoS here is my advice.
Setup your Nortel phones to mark voice traffic with some type of CoS markings.... 5 would probably be the most commonly used one for VoIP.
Set your switchports that connect to the IP Phones to trust these markings.
Then create some class-maps on your routers to match your voice traffic based on what you have classified them as (cos 5)
Then crate some policy-maps and use LLQ to give your voice class-map priority over anything else being sent over the WAN.
So it could look something like this;
On your access layer switch ports that connect to your IP Phones configure them to trust to cos from the IP Phones
(config-if)#mls qos trust cos
Then on your routers
class-map VOIP
match cos 5
policy-map critical
class VOIP
priority 2000
SO here we matched traffic with a cos vale of 5 (voip traffic in this case) then created a policy map to use LLQ for voip traffic with a strict bandwidth priority reservation of 2000Kb.
Then apply that to your WAN interface
(config-if)#service-policy output critical
So that pretty much guarantees your VoIP traffic 2000Kb of bandwidth no matter what else is happening on the WAN.
Now obviously you need to tailor this to suit your needs, and you would also need to create other class-maps for your other network traffic and probably implement some type of queuing for your non voip traffic (CBWFQ would do)
