Managing dialer on Lockscreen - TC75 L

I'm looking for the best way to approach a situation I have with a couple of customers....TC75 Lollipop AOSP

In Lollipop, the lockscreen has a phone icon in the bottom left corner which allows you to call anyone.Customers dont like this their applications are locked down so that only certain number can be called

Can anyone recommend the best way to manage the calls that can be made by the dialer from the lockscreen?

A custom dialer?

Is there a way to limit calls the dialer can make to only those made from the phonebook (including 999)?

We need to keep the lockscreen as the customer wants to use it for notifications...

All suggestions welcome - thank you kindly!