Magnus,
I am not sure what a 6731i phone is, but I assume it is setup as SIP so we will go from there.
You are correct that SIP will not support DSS/BLF functionality, but it should support MWI so look under the System > SIP Phone > {9xxxx} > Configuration > MWI > Accept MWI: YES
If they use the NuPoint there is a transcription service that Mitel works with that is supported; otherwise I don't know of one that is supported for the built-in Voice Processor. This doesn't mean that you can't get one to work though; it just means that Mitel hasn't sorted anything out for this functionality.
Sorry,
TE