I've been an rsync.net customer since Mar 2013. I sent a fairly straightforward administrative support request once back in 2015 and got a response back within a day or so. Haven't needed any support other than that; their service supports standard sftp / sshfs with the usual tools. The CEO also occasionally chimes in on comment threads on HN.
I believe based on their pricing structure, 24/7 phone support starts at the 100+ TB tier, which should work out to about $1500+/mo. But all of their standard accounts should provide support, within reason. See https://www.rsync.net/pricing.html.