This is Medicin San Frontier and is generally the number they use for existing contributors. The don't leave voicemails and will immediately try another contact number for you.

I called back number (02) 9023 1102 rang 3 times then goes to busy tone.

The medicins sans frontieres comment sounds legit, I've got a missed call and am an existing contributor (and I know they do like to call people to try to get them to donate more). Didn't call back to check though.

I have been having issues with ny connection and have contacted them, so maybe legit?

I discovered it was Medicine Sans Frontier - charitable organisation. Pity they dont leave a message to that effect.

I got a message to text saying 'hallo hallo hallo' - maybe it was the Metropolitan Police c.1953.

Caller did not leave a message. No idea who's number this is.

they rang and did not leave a message

called but did not leave a message. Do not know who this number belongs to.