Complex sentence: If the network does not support A5/1, or any other A5 algorithm implemented by the phone, then the base station can specify A5/0 which is the null-algorithm, whereby the radio traffic is sent unencrypted.
Split into simpler sentences: If the network does not support A5/1, or any other A5 algorithm implemented by the phone, then an unencrypted radio connection can be used. However, the base station also has the possibility of specifying A5/0 which is the null-algorithm, whereby the radio traffic is sent unencrypted.