If you're not tagging the VLAN on the handsets, then what's the tagging on the switch for?

Shouldn't you be tagging the VLAN on the handsets?

Though the fact you can see the DHCP offer is strange.
I've seen samsung phones not get DHCP correctly when there was a missconfigured DHCP option.

Also the Samsung ITP-51xx won't work with DHCP when on a tagged VLAN.

Were all the handsets working before the upgrade?