Now you can keep your existing phone number for FREE 
So I think it shouldn't be a problem.
But still, please ask your alarm system's technician if Unifi network can support your cctv model.
Tell your technician you can keep existing phone number.

I have no idea when usage policy will be implement. Sorry

Unifi's phone will not work if you switch off the modem.
So you'll need to leave the modem on 24/7.
Btw, you are interested to apply new line issit?
If yes, you can apply streamyx first and upgrade to unifi later.
then you'll have two numbers (1 copper and 1 voip) in the future.   
but it is abit mafan 

Hi,

Yes, the technician needs to go your place to install for you, but it is free
You will be given 1 free wifi modem and cordless phone.

For unifi part, it is still ZERO entry. You don't need to pay for the equipment. 

Hi, I had make confirmation with TM.
It takes 3-5 days for streamyx installation done.
After 1 month of streamyx activation only can upgrade to Unifi.
The cheapest streamyx is 384kbps, but it is really slow.