To test this, I've configured the phone to use a static IP and I will follow up tomorrow.
Meanwhile, I wonder, wouldn't the phone request a DHCP renewal every now and then and thus would not lose it's expiration? MY router doesn't have a DCHP lease time option so I don't know if it's hard coded to 12 hours or not. We'll find out soon but the phone should be renewing its lease, no?