I would say SC
AM not spam and almost ceertainy not genuine. Why would BT call every customer in an area when there is a problem? It will cost time and money to make hundreds or thousands of calls. And if you, plus others started running speed tests you would worsen the situation!
The number 00310800800150 contains the BT contact number of 0800800150 so they are possibly hoping that is all you see! If it was a true international call it would probably have have been 0031800800150 ...
It might have been useful to find out which website they were to point you to although not
to actually try it.
M H C
taurus excreta cerebrum vincit