How to react once you discover someone plagiarized your call for papers?
The call for papers is normally written by the guest editors of the special issue, so they are probably the ones at fault here.  If, as you say, that other journal is reputable, then its editor (i.e., the usual editor, not the guest editors of the special issue) probably doesn't want to be seen as complicit in such a disreputable activity as aping another journal's special issue selection and plagiarizing its call for papers.  I'd fire off an e-mail to them describing what has happened and asking politely but firmly for an explanation.  If they shrug you off, then the journal is obviously not as honourable as you had imagined, and you can then decide whether to drop the matter or to escalate it (for example, by getting in touch with the journal's publisher, or by publically naming and shaming them).  If you do choose to escalate, you might want to first discuss this with the editor and/or publisher of your own special issue's journal.