new 3CX system using Cisco switches and Watchguard firewall config help

[bfordz]

We just had a 3cx on-premise server installed this past week; it's running on Debian, sorry I don't have the version at this time. I'm waiting on my logon credentials.
We're using Grandstream IP phones; GXP2135's and GXP2170's.

I'm using a Watchguard M370 Firewall which is setup to provide DHCP over VLAN 50 on Interface 2 for my phone network.
My firewall and 3CX server are connected to two Cisco Catalyst 2960x switches.
Interface 2 of my WG is set as a VLAN, with VLAN 50, using 172.16.50.1/24 network. The connections on the Cisco switches are set as Trunk ports with VLAN 1 and 50 (1 - trusted Pc network; 50 - trusted phone network) for the Watchguard, 3CX and IP Phone connections.

I'm a newbie to IP phones and how I need to set them up on our network.
The problem I'm having is the IP phones won't stay connected to the switch because they seem to randomly grab an IP address from my VLAN 10 not the VLAN 50.

My question is, can someone point me in the right direction as to the correct setup (settings) for a Cisco switch with Grandstream IP phones and a 3CX server?
Is it possible I have a setting in my switch causing this or should I be looking somewhere else, I'm not a Cisco specialist so I'm at a loss for where to look.
Our full install and cutover has been postponed until I can get this straighten out.

 

[jcostlow]

Is the VLAN ID set for the phone port on your phone provisioning tab as mentioned here: https://www.3cx.com/sip-phones/vlan-configuration/

It sounds like they are not being told to use VLAN 50. LLDP or Cisco Discovery Protocol (CDP) needs to be turned off and set option 132 on your DHCP server.

 

[bfordz]

jcostlow,
Thank you for your reply and the link.
After speaking with a Cisco person, our vendor who is doing the install and now your input it's interesting but I've learned a lot about the IP / network installations.

No, the VLAN ID is not set on the phone port of the phone provisioning tab, also our vendor doesn't use any of the DHCP scope options (150/132).

The link you provided was the kind of information I was looking for, Thank you! It was helpful in understanding how this works / how to setup.

Cisco suggested having LLDP turned on, which at this point it's a good thing I was't trying to do both LLDP and Option 132.