Two vlan on lan1

Good morning, i'm new on firewall zyxel and i have a problem that maybe simple for you but i don't know how to solve. Sorry in advance for my bad english, i'm an italian technician. I've a company with about 30 pc and 25 voip cisco. The pc get ethernet connection from the voip phone cisco. There's a managed switch cisco that have vlan40 and vlan20 configured (vlan40 for all phone except 4 that work with vlan20. From when i installed the usg40, all pc and all phone with vlan40 work very well but 4 phone with vlan20 don't work (but the pc attached to that phone get internet normally). What i have to do is to have vlan20 and vlan40 configured to work with lan1. Can somebody help me with this?

Thanks