handful phone deregister and then re-register

[redrobish]

Hi, I got 7940 & 7960 phones on site B. The Call Manager is on Site A.

problem:

On site A- no phones restarts

On site B- a handful of phones deregister then re-register...

check all switches, no errors, not reboot,etc...

any idea where to check?

[thisisshanky]

is your wan circuit clean, no errors, link flaps etc?

[redrobish]

how to monitor that? any command?

all my switc is cisco 3560...

sorry, im new...