I'm still sweating at the idea of this, it could have turned out so badly especially considering we were on the 3rd pitch and probably around 40m high. Communication is so important and I have realised that even such a simple mistake could potentially lead to terrible consequences. It is important to always have a set protocol of communication with your climbing partner, and making sure it is clear. We have now changed "safe" to "I'm safe", which makes it much easier to distinguish, but maybe changing it completely to "off belay" as I saw suggested in one of the other incident reports may be even better. Alternatively, taking a phone up so you can call your climbing partner may be useful although obviously you need a free hand for this so might not always be possible. From now on, if in doubt, I will always make 100% sure that what I heard is right before acting or at least assume it's the one that will at least keep my climbing partner the safest.