This is now happening much less often, I am glad to say. However, I am still unable to take advantage of the "Push Service" for the reason that WHEN registration is dropped, the indicator on the phone still shows GREEN, as if nothing happened. However, outbound calls are REJECTED by Voip.ms server, for the reason of NOT REGISTERED, and inbound calls are unsuccessful. That makes Push Service useless to me, however useful it would be for me in terms of lower battery and data use.
@Zoiper, please, find some way of checking the integrity of registration status, as this false indication is really troublesome.