I friend of mine says that whenver I am talking to someone over phone, my whatsapp status shows offline i.e. neither it shows me online (since I am not using whatsapp at that moment and talking to someone on phone) nor the "LAST SEEN" status is available. And he can even tell me the exact duration of time I spent talking to that person. Is this possible?? Is this a feature of whatsapp?? because as far as i know, no such case exists and if I try to view the status of a person who is currently talking to someone over the phone then obviously the "LAST SEEN" status for that person is available. Is the friend of mine using some software or app??
Your data connection is probably being lost during the phone call. Thus, whatsapp gets disconnected.
CDMA networks (Verizon, Sprint, and others that do not use a SIM card) do not work with data and voice at the same time when on 3G. Voice takes over, so anything using data is disconnected. Verizon is working towards this feature, called svdo (Simultaneous Voice and ev-DO data), and according to that page has one device that uses it, the HTC Thunderbolt. Similarly, Sprint also has the HTC Evo 4G LTE that can use it. Almost all other devices, though, do not support this.
However, LTE (4G) does allow for voice and data simultaneously, so certain devices that are LTE capable and are connected to LTE can maintain data connections during a phone call.
In the end, if you have service through a network without SIM cards, and you aren't connected to WIFI or LTE, then you will lose data connections when you make or receive a phone call.