When a prepaid caller with no balance is trying to issue a call the network dully rejects the attempt and the B-party remains unaware of the missed call. Here comes Opencode Mobile Beep Call which will push a poke call to B-party (micro call that will appear as missed call). In almost 40% of the cases the B-party will return the call thus a sizeable portion of the rejected by the networks calls will be recovered.
B-parties can always receive poke calls no matter if they are on- or off-net subscribers, pre- or postpaid, in roaming or not, etc. A-parties will be unable to use the service if they are off-net or are explicitly deemed non-eligible to use the service.