"But, surely, Christians are effectively calling Muslims liars because Christianity does not recognise Mohammed as a prophet of God!"

Not at all it's more like they are calling Muslims mistaken , unenlightened , wrong – delusional if you like!- but not liars . There's a distinction between those who tell an untruth because they're mistaken and those who knowingly tell an untruth.

For example . Somebody above wrote to say that my account of a conversation with an old Muslim friend was "dubious". If I had been making it all up , that would make me a liar. But if I genuinely believed that this conversation had taken place, but was mistaken in my reporting of it , or if my memory of the conversation was only the memory of a dream I once had , that wouldn't make my recounting of the imagined/delusional conversation here a lie.