Always put primary or main numbers in phone. regardless of if it's a cell phone or not, treat mobile as the secondary phone and put numbers in there which additional number we want to keep track of.  Also use the notes field next to the phone number to store any information about the phone (who it belongs  to, if it's valid etc) NEVER and I mean NEVER enter words or letters into in the phone field doing so will be the system will no longer be able to send text messages to that person which means they will not receive any receipts or alerts.