Your channel only contains the optional dialled digits component. It should include the technology and device components, e.g. SIP/ITSP1, DAHDI/g2, or Local/…@context
Unless it has been added recently, you cannot address specific members in a queue, but, in any case, you would need explicit dialplan to parse the two parts of your extenion. Also, whilst I’m not sure that it is technically invalid, use “@” in an extension is likely to cause confusion.
If you want to call a member that is an always on agent, you need to define an additional queue with just that agent as a member. If you want to call a member to which you dial out, you need to address them directly, not relative to the queue. This assumes that addressing a member of a queue hasn’t been added, but I’m not aware of significant new work on queues, and this would be a low priority compared with other queue issues.