The default setup on the Zoiper phone is making use of UDP (it was setup making use of the Zoiper QR code: on FreshPHONE TSM).
However, according to Zoiper's setup instructions, it is stated:
Please advise what settings should be used as it might also impact battery usage..
These are merely transport protocols, they should not affect battery life. We generally say use UDP because as the nature of the protocol is to fire and forget, it works well with real-time communications like VoIP. You may use TCP but you will not see any significant benefit to doing this unless the network you are connecting to has strict firewall rules in place which do not favour UDP traffic.
At the end of the day you can use either it will not make a difference.