Vivek is correct that calls already setup will continue if the gatekeeper comes down. In its strictest sense this applies to gatekeepers which allow either direct call signalling (everything but the RAS signalling goes directly between the 2 clients)
and gatekeeper routed signalling (where everything goes via the gatekeeper).
The reason for this is that once the call is setup between 2 IP phones the H.225.0 signalling channel is not required (supplementary services aside).
However there are some configurations in which a gatekeeper failure will cause calls in progress to fail. The ones that I can think of are below. Please also remember that the scenarios I give below may or may not cause calls in progress to fail it will usually depend upon the way that the gatekeeper or client or gateway has been programmed to act in this situation. Some of the gatekeepers that I have worked on will clear a call in progress in 1 or more of the scenarios below while others will allow the call to continue:
1)calls between a PSTN to H.323 gateway and an H.323 IP phone when gatekeeper routed signalling is being used. The H.323/H.225.0 recommendations say that when a gateway is involved the H.225.0 call signalling/Q.931 channel must remain open (when the call is only between H.323 IP phones it can be left open or closed it is an implementation decision). Some implemenations when detecting that the H.225.0 channel is down and that a gateway is involved will clear the call.
2) some gatekeepers which allow/force gatekeeper routed call signalling also allow for the routing of the H.245 logical channel 0 to be gatekeeper routed.
A gatekeeper failure in this scenario may very well cause a call in progress to fail.
Also whether the gatekeeper allows/forces direct signalling or gatekeeper routed signalling the
endpoints will usually have to re-register after a certain period of time specified by the TTL in the original exchange of registration messages. If the TTL expires and
a client cannot get a response from the gatekeeper to re-register then they will not be able to make any more calls. Even if pre-registered ARQ was in use most H.323 clients use the gatekeeper for directory services i.e. phone number to IP address lookup.
There are ways to use a secondary gatekeeper if the first gatekeeper
failed. This can be done by having the client configured with the secondary gatekeeper IP address or
by the gatekeeper specifying an alternate gatekeeper IP address to use if required in the registration and/or admission messages.