Most likely VOIP working over Mobile Broadband wouldn’t work all that great if at all… this is because VOIP needs a reliable high-speed connection to work and cell technology would have a hard time keeping up with the demand. If it would work, most likely you would drop calls frequently.

But, in theory, you could share your Sprint connection with the router by enabling Internet Connection Sharing in Windows… (look in the help files for how to do this) and then connecting a cable between your computers Ethernet port and your router’s IN port (that your cable modem used to be connected to)