Hello Mikey,
I am not familiar with the service or equipment offered by Phantom, but having looked at their web site it seems they offer two types of system.
The basic does not continually monitor your tracker, but they can interrogate on demand to find out where it is.
The second service is the Target ProActive, where they do monitor the tracker and will respond when an activation signal occurs.
It does not say how the signal changes when an activation occurs, but it might not recognise loss of signal as an activation.
You must also consider that the tracker may be in a signal dead spot, or the notification messages that Phantom send out may not be received by your by your mobile phone again because of signal problems.
You should discuss with Phantom or your supplier about the service you have contracted to, and preferably be negotiation try to resolve the issues.
If that fails, and you are still convinced that your supplier is in breech of contract, then contact trading standards or a solicitor who will advise professionally.