I phone sent an audio message without me knowing.

No idea. But mine phoned my daughter at one in the morning not long ago.

I was in bed asleep so I know it wasn’t something I did. There was just the ring tone. Then she had to phone me back to make sure I was ok.

Sleepless night for both of us?

DS phoned my landline from his mobile this morning. I could hear someone else talking in the background during quieter bits of our conversation, although not constantly.

Could it have been caused by "atmospherics" ?

Thank you for replying. Lathyrus . Your uninitiated phone call is not unlike the audio text. Septimia that sounds like your phone picking up on another conversation.
What worries me is that our conversations can be sent to anyone without our knowledge. If I had been holding my iPhone I would simply have thought I had touched something be mistake.

Check if your phone has a voice function,

The Alexa answers me quite often when it thinks I said "Alexa"

Was your iPhone reasonably near when you were speaking to your husband?

My best guess would be that you said something to him which Siri (the voice-controlled personal assistant on your iPhone) picked up as “call [whatever your friends’s name is]”. It called her, she didn’t answer and whatever you were saying to DH went to her voice mail.

If you said say, I’ve just called the doctor, Siri might pick it up as "call Dot."

If you only have only one person with whatever name Siri thought it heard in your contacts and only one number for that person it would make the call. If you have more than one person in your contacts it could chose from and/or more than one number for one person it would ask more questions to determine which number to call.

You can switch off Siri in your iPhone settings.

Thank you so much Marlene for explaining all that. I am going into settings to see if I can switch it off. I haven’t used Siri for ages as it really got on my nerves. I was calling out to my poor hard of hearing dh so the phone could have picked it up. Thank you again.

Well I had turned off Siri I also turned off voice memos so not sure if I have helped but just trying everything. Your explanation has to be right as nothing else makes sense. Thank you again.