If this had occurred on an analogue system – like TACS 900MHz – it would have been suggested a case of frequencies separation between mobiles being very close, thus poor radio planning. Of course TACS did use localised encryption is voice transmission.
In GSM Key Ciphering (Kc) along with allocation of TMSI is expected to eliminate cross-talk problems like you mention.
I am not sure though… but sounds like an internal network issue.