In reply to Jamie Wakeham:
> They reckon it'll use about 100MB data a month
What update rate is it transmitting at? And what message size?
I have some cheap trackers that send about 150bytes per update. At a 5 minute update rate, that's 43200 bytes per day, or 1.3MB per month. That suggests once every four seconds for yours.
I use GiffGaff SIMs for my trackers. No 'bundle', just a simple, PAYG, which doesn't expire. At 5p/MB, that's 10p a month for my trackers at 5 minute report rate.
You can usually send a command SMS to change the update rate; if the dog goes missing, send it to update much faster, so you can live track it.
I will look at the M2M SIMs suggested, as most of them are multi-network, which would be useful for better network coverage.