Mobile communications system insensitive to false frame sync

In a mobile communications system, the base station sequentially scans speech channels and a control channel to monitor call status and detect a call request from a mobile station containing a frame sync and an encoded user's identifier. A successive bit sequence of the call request signal is stored into first and second buffers (13, 14). A first count value (11) is derived to represent a count of bits stored into the first buffer (13) from the instant an earlier one of two successively sync codes is detected and a second count value (12) is derived to represent a count of bits stored into the second buffer (14) from the instant a later of the sync codes is generated. When the first and second count values respectively reach a prescribed value, each of the buffers is triggered and the bit sequence in the triggered buffer is loaded into a decoder in which it is decoded and checked against a stored version of user's identifier. If there is a match between them, a signal is transmitted to the mobile station indicating that the call request is accepted. In a modified embodiment, a single buffer is provided and a count value is incremented in response to each of two successively detected sync codes if they are spaced apart a longer interval or in response to an earlier one of these sync codes if they are spaced apart a smaller interval. The bit sequence in the single buffer is loaded into the decoder when the single count value reaches a prescribed value.