Yes, Nokia 8810 4G does have the VoLTE support. You can make calls via VoLTE enabled network easily.
Nokia 8110 4G comes with 1500 mAh battery to support the heavy requirement when you want to use the vibration function of the phone. The vibrator is strong enough even if it is deep inside your handbag.
Yes, Nokia 8110 4G is a dual sim mobile phone. You can use one 4G sim card in one slot and another slot will be on the 2G network.
Nokia 8110 4G runs on KaiOS which does not have the WhatsApp support. Nokia management has updated that they are working on the same but there is no definitive promise made for the same.