While on a phone call using the Virtual Office Mobile app in data mode on a Sprint or Verizon smartphone, a call in to the cell phone's direct phone number (DID) forces the VO Mobile call to drop.
Virtual Office Mobile on Sprint or Verizon
Use Cellular Assist so that the cellular connection is handling inbound and outbound calls rather than the mobile data or Wi-Fi connection. Alternatively, use a Wi-Fi connection for VO Mobile data phone calls.
This is a known issue with older Verizon and Sprint smartphones, which are unable to simultaneously handle cell phone and mobile data activity. These phones are 3G pre-LT/LTE models, typically up to and including those debuting in 2012. This covers Android and iPhone models up to and including the standard iPhone 5.