This question is lacking a lot of details to give a definitive answer, but here are some suggestions.
- Look into the DHCP server register, there should be an entry with possibly a recognisable identifier.
- Snif on the DHCP server network link and reconnect or restart the IPBX. Then look at the DHCP transactions.
- Snif on the IPBX network link and reconnect the IPBX. Then look at the DHCP transactions.
- Snif on the port of the network switch and reconnect or restart the node. Then look at the DHCP transactions.
