How to prefix subject line by rule?

I’m coming from PostBox. Have purchased em client licence. A copy of my sms texts arrive by email. voip.ms (my voip provider) sends the email with the subject line “Message received from 123-456-7890…”. In PostBox I had a filter, based on the phone number, that changed the subject line and added the person’s name.

So after filtering (or a rule) the subject line would read
“Text from Joe Smith - Message received from 123-456-7890…”

How can I do this in em client? In PostBox this was a “add prefix to subject” option among the features a rule could perform.