The advice is based on my years old experience that if something was not perfect in a SIP message, the ALG in RouterOS was unable to recognize it as being a SIP message and did not forward it at all (the message was coming from the exchange side in my case, and the exchange vendor has fixed this particular already; in your case it seems that it is something about CSIP Simple given that another softphone running on Windows experiences no issues, or so I've understood from your post at least). Other vendors' SIP ALGs used to have their own issues.
Another experience says that as a VoIP provider, you cannot rely on SIP ALG to exist and work properly at each client side. So most VoIP providers' exchanges deal with NAT at client end by means of ignoring the RTP socket address indicated in clients' SDPs, learning from what socket the RTP from the client actually comes to the socket on a public IP which they have indicated in their own SDP, and sending the RTP for the client to that learned socket. This approach has some drawbacks too, but in general it has a much higher success rate than depending on clients to have a working SIP ALG which also plays along with their CPEs. So it should be safe to switch the SIP ALG off at Mikrotik.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.