In practice, most businesses are simply handling this on a case-by-case basis since COVID is a temporary problem. Most home networks can handle the extra load as long as the employee makes sure that other latency-sensitive traffic, like gaming or video streaming, is kept to a minimum during the hours when they need to talk. If some home routers develop problems, IT staff simply build a queue and handle those one at a time. Sometimes they'll be able to access the router remotely with the employee's permission, sometimes they'll have to walk that employee through configuration steps to fix the problem. Sometimes the employee will just have to live with it unless the company springs for a new router or a higher bandwidth tier from the employee's ISP.
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The benefit of a cloud-based system is that all messages dropped by prospects can be converted into digital formats. Virtual voicemails can be accessed by agents anytime anywhere. They can also be added easily with other data to generate reports.
Businesses taking advantage of BYOD capabilities will usually set monitoring/security/data ownership and access policies to protect sensitive business data if/when these devices are taken out of the standard office environment.