SIPTRUNK has several plans and flexibility within each plan. You can have:
An unlimited trunk, which allows for unlimited calling. So, you could have 6 trunks each with a single DID and call path. In this case, you would have set each trunk up within 3CX along with its respective DID on the inbound rules and then use the failover rules within your SIPTRUNK portal to handle the rollover.
Looking at the above, the "primary" is a trunk. If it were to be busy, you could then set-up the secondary to point to the next trunk and then repeat same for all trunks so that you create a circular huntgroup.
Using the same unlimited trunk, you can also set up the number of call paths, so that only one DID is needed; yet 6 calls can be handled at a time. Or, you can also add additional DIDs which would then share the 6 calls paths. There is no need for failover if there is call path availability. However if all 6 calls paths are in use, they may allow it and then send you a message that you have exceeded the plan and should consider upgrading. Otherwise, the caller will get a busy.
The unlimited simply means that you are not bound by a set number of minutes and in exchange you are billed a flat-rate regardless of whether 2 minutes or 20,000 were used in a billing period.
It depends on how you are configured at SIPTRUNK that determines what is needed at 3CX.
Unless your have specific needs for that many trunks, I suggest you ask the reseller to consolidate the 6 DIDs under one trunk and then set the trunk to accommodate the desired number of calls/call paths. It is the same thing financially, just easier to maintain from your perspective. Where you see the 1 in unlimited channels, you can use the down to select how many are desired.

