The criticism usually is that you're giving your SIP credentials to a third party when push is involved. However, Acrobits can give you the software to admin your own SIPIS server, where you live, if you're willing to put in the work and keep a server running 24/7: https://doc.acrobits.net/sipis/installation.html. Also, visit https://forum.acrobits.net/viewtopic.php?f=1&t=101#p391.
And, in that case, it's not all that secretive and spooky.
OBi200/202 Freephonline PDF guide (version 1.51x) can be found here.