Quick answer: Onyx Slots keeps live chat open around the clock, its email team replies within 24 hours on weekdays, and there's no UK phone number to call.
I've live-chatted, emailed, and dug through the help centre at Onyx Slots more times than I'd like to admit, chasing a slow withdrawal at midnight more than once. Contacting Onyx Slots takes minutes through live chat, longer through email, and there's no phone line to fall back on.
Onyx Slots tucks live chat behind the "Help" tab in your account menu. I opened three separate chats over a fortnight: one about a stuck deposit, one about wagering on a reload bonus, and one just to test response speed at 2am. Every agent replied inside two minutes, and the agent handling the deposit issue solved it in the same conversation, no ticket number, no callback promise.
Onyx Slots saves a transcript of every chat, so you can screenshot it if a dispute comes up later.
Live chat stays open around the clock. Onyx Slots' email team replies within 24 hours on weekdays; over a bank holiday weekend, my query sat for closer to 36 hours before someone answered. Land your question on a Saturday night, and only live chat gets you an answer the same day.
Most tickets I've raised with Onyx Slots customer support fell into three buckets: verification documents, bonus terms, and withdrawal delays. A Faster Payments withdrawal sat "pending" for four days until I raised it through live chat; the agent checked the payment queue and cleared it within ten minutes. PayPal withdrawals cleared within a day of approval in my experience.