The Wireless M-Bus standard does not apply any CSMA algorithm. The Multical21 is a pure transmitter using C mode, like “transmit and pray”. But it transmit very often, so the probabilty is still very high to receive it, because of the limited duty cycle of the transmitters. The RC1180-MBUS3 can be jammed by strong transmitters at same frequency using 802.15.4, but the module will not receive or decode such data as the preamble and synchronisation word is different. The frequency you mention is same as wMBus mode S, and the concentrator to meter communication for mode T2. But mode T/C from meter is 868.95 MHz, so there will not be a direct collision.
Last Update: November 9, 2017