There are numerous USB headsets to choose from, but the following criteria are not obvious on the first glance:
Must be compatible with Ubuntu Linux
Sound quality/enhancement optimal for online conferencing i.e. human voice transmission (remote sites can still have effects which at least should not be emphasized by the device. If it could in some intelligent way compensate like dump glitch unpleasant sounds that would be really awesome)
You should be able to wear it for many hours a day if the situation requires it, that is, the design must be at least partly optimized for ergonomics. Specifically, not too heavy and with soft padding. Who knows, maybe somebody has designed padding with antibacterial anti-transpiration capabilities.
Active noise cancellation in the speakers is not a requirement as long the assumption is valid that it will be a significant price add-on factor. Noise cancellation microphone would be great though.
Keywords "professional call center" might admittedly way too broad criteria which means here in first place a requirement for good material quality and device durability.
Wireless connection is not a must have. But wireless via Bluetooth is possibly not acceptable if could influence the quality of communication.