Questions about DHCP

[Alcatom]

Hello everyone,

I got a lot of informations about Alcatel thanks to this website but I have some questions about DHCP for which I can't find answers.
OXE Version: R100
Omnivista 8770
Phones: 8068s, 8038...
DHCP config: 3 differents subnets/ranges

I'd like to know how can I release a DHCP lease from Omnivista or OXE CLI? I tried to remove the IP address from Omnivista, reboot my phone, set a static IP, reboot and re-define a dynamic address but it get back the last one again. I saw that leases are defined at 3600s, is it normal/best practice?

I tried the "dhcplease" command and it looks like I have 0 leases on 2 subnets (it can't be possible because I have many phones with an ip address in those and still working) and some leases in the third one.
After many tests, it looks like new phones which never got any IP address can't get one, phones which already have an IP can keep it by sending "keepalive" packet over the network.

We've already made a lot of tests and packets analysis with wireshark over our network, we saw the packets sended from the phone mac-adress, get relay to the ip helper-address but it seems there is no answer from the server.

Sorry if my questions aren't really clear, don't hesitate to ask for more details.

Thanks
Tom