Most call centres would have the number hidden - call centres that call out, rarely have the facility to take incoming calls. The chances are that it is a mobile numebr, but that doesn't rule out someone trying to sell you something!
each network has it's own starting code to their number, eg, a contract 02 starts 077 but a pay as u go 02 starts 078, if you find a site for these numbers, u can check with the network what the number is. (I think)
Actually you are half right Dot. It was indeed once the case that you could identify the network by the start of the mob no ie 079 for T-mobile but with the freedom to port to other networks and retain your original number, this is now a thing of the past. So it makes it harder to identify which network the number belongs to.
In answer to your question smileygirl, I agree that if the call is important they will call back. Contact centres usually withold their number but on occasion will release a number which is transmitted but which you will be unable to call back on and successfully speak to anyone. You will more often than not hear a pre-recorded message along the lines that a marketing company attempted to contact you but were unsuccessful and will try again. If the number you have is an eleven digit number and begins with 07, then the chances are that it is a mobile. You could always try again at a different time of day or wait for them to call back. There is also the possibility that the call wasn't intended for you and was simply a wrong number.