Hello,
We refresh the registration because on iOS, we can't be sure that the socket we're using is still alive when coming back from background. This is especially true if your SIP server uses UDP, where iOS will systematically kill the socket. So we refresh every time you come back from background.
On android the problem doesn't exist because the refresher process is always in memory and active, so we can do however we want (and Android has a proper network stack that keeps sockets opened).
On the other hand, if you are using only TCP or TLS connection (which are kept opened by iOS), I believe you could bypass this refresh. So give it a try.