Managed Services Market Size, Share & COVID-19 Impact Analysis, By Services (Voice Service, Non-Voice Service, and IT Managed Service), By Function (Finance & Accounting (F & A), Marketing, Procurement, Supply Chain, Human Resource, and Information Technology (IT)), By End-user (Medical, Financial, Government, Audit & Consulting, Corporate, Telecom, and Insurance & Re-insurance), and Regional Forecast, 2021-2028