I worked with the Nokia M-BUS many, many years ago.
My recollection is that it was a synchronous, multi-drob bus, with a clock line and bidirectional data line.

We were working in conjunction with Nokia-Mobira (as they were called then), and managed to obtain an "M-Box" from them to convert between MBUS and RS232. It contained an NEC processor to do the conversion - it wasn't just a simple matter of a MAX232!!

The M-BUS used a packet-based protocol, with addressing & collision detection, IIRC - not text 'AT' commands.

But maybe it's different these days?