You should be able to Google it.
Or I, or anyone else, can grab it out of our own. I can do it tomorrow
Or, deploy the OVA as a single box on a new mgmt IP and export it from there if you wanted to get it in the next 30 mins without searching too hard
What FQDN do the phones use to hit DES? If it's like 'des.Avaya.com' then just punch that in your browser, inspect the cert and export each cert in the chain.
You can take many PEM CA certs that form a chain and paste them into a single file in order of 'furthest from root CA' to 'root CA' and import that as a trusted CA cert. You probably won't have to as I only have to do that when adding an identity cert from GoDaddy or Digicert, so you could probably just go to des.Avaya.com, check the cert in your browser and export just the root CA and that might be enough to replace it.
Are you using DES? Is there a reason the SBC needs to trust it? As in, perhaps it's reverse proxying the outward phone traffic to DES and needs to trust that DES cert?