Alec, it seems as if you need to take a photograph of the cabinet in question and then post the image here, for you have us all well puzzled! 
I agree. Without seeing a picture then its almost impossible to say. They design of the air vents is often one of the biggest clues. VM cable cabinets are usually grey, but they also have a large amount of green cabs that can look like BT PCPs with vents on.
The mobile provider cabs come in a wide variety of designs depending upon the provider and are also usually grey, but some providers also tend to make a huge effort in disguising them and blending them with surrounding area. Theres a website somewhere which I saw a while back which has some fantastic examples of (what iirc O2) used to do to disguise and blend them into the background.
