I am also planning to offer such verification services as I already have plenty of numbers which are capable of receiving both sms and voice calls. I plan up a versatile services which covers members here in this forum round the clock. If we both from different time zones and you like partnering with me kindly let me know.
Covering all time zone must be a right idea, but for that going for a website with fully automated process must be a right solution. If your budget permits, build a service with fully automated processes rather than relying on manual competency.