I know I've said this before, but switching from software #VOIP phones to a hardware one - even a generic SIP phone, as opposed to one tied to a particular provider - makes inexpensive VOIP service just so much less of a pain in the ass to use. this is especially true for incoming calls.
Android SIP phone apps all suck. and #SIP phone apps on a laptop running Linux can only ever be as good as Linux's audio I/O handling capability, which as we all know is not good, and also completely depends on what you may or may not be doing with your headphones, laptop, etc at any given moment.