BT Voyager 2100, 2110, 2091, 210

By default the above routers are shipped with ICMP pings turned off. There are many reasons why you want your router to be pingable and it isn't always the security risk that some sites may make out.
In fact making your router unresponsive to pings can actually cause some problems since turning off pings, also turns off ICMP which is a valid internet protocol and can cause problems with PMTU and some sites may not display properly. (See MTU Problems).

The feature is a bit hidden away and not in what may be considered a logical place.
To make your router pingable access the web interface :-

http://192.168.1.1

Advanced >
System >
Remote Access >
Tick the box marked PING.