Our client would like to setup his network wirelessly and wants to be able to use sipp phone's anywhere on site.
If we were to setup RB433 boards and use dip pole antenna's. When a user is on a call will the call drop when handover between 2 antenna's occur, if the correct configurations are in place?

It should be fine - but it is likely that the user will experience a "stutter" when the handover occurs -depending on your set up.

This much i know for sure: we can switch BGP paths over our interconnects (thus changing the path taken) without the customers noticing No reason you couldnt do the same.