DHCP issue on Yealink phones with HP Switch HP2920

[tsukraw]

Looking at your 2920 config you have this list...
vlan 50
name "VLAN50"
untagged 4,10-20

Where is the tagged?
If you are wanting to put the phones on a tagged vlan which is what it sounds like you would want to have those ports "Tagged". Here is what i would normally see

vlan 50
name "VOICE_VLAN"
untagged 4,10-20
tagged 1-3,5-9,21-24,A1-A2,B1-B2
ip address 20.1.50.10 255.255.255.0
ip helper-address 20.1.10.240
exit

If you are unsure how to get this going on the 2920 PM me and i will give you a hand. Got probably hundreds of HP switches out there so shouldn't be a issue what you are looking to do.