It is so frustrating when you get a missed call from somewhere you don't know and they do not leave a message.
Had two recent calls from this number when i was out. When called back there is an educated female American Phillippinno voice (as opposed to their front line gooks who are normally half incomprehnesible in terms of their English) saying they called about "a simple household survey" and that "it is nothing to worry about"
Would assume these are data harvesters and data resale merchants of the worst kind and that if you give them the time of day you will then be plagued with numerous further sales calls from unwanted UK and overseas sales scammers of every kind as has happened to me in the past despite my TPS registration.
I suggest you call them something very rude, slam the phone down on them and then report them to both Ofcom as a Silent Call and the ICO as an unwanted sales call. You cannot report them to TPS as TPS requires the name of the calling party