How to tell if VOIP is connected to Cellular phone line?
Enthusiast - Level 3

Not sure if I am using the terminology correctly but hopefully someone understands what I am asking here.

I would like to know if there is a way to determine whether or not a cellular phone line on my unlimited family plan which currently has (4) four active cellular phone lines, has a SIP/VOIP (Google Voice, Discord, Skype) number associated with one of them. Obviously if I was to ask the family member whose phone is in question about this I am not too confident the answer will be all that truthful. Unfortunately.  Which is why I would like to know if somehow this can be found out maybe by tech support if not on my own?

FYI,  Any of the 3 voip service apps I mentioned above are not currently installed on said phone line. 

Thanks in advance for any information or answers I may get here.