Telephone on Saturday

Hello Rabbi. For a long time I have been debating the issue of electricity on Shabbat, and I have not been able to convince myself of the prohibition of using the telephone on Shabbat. All the methods that have been said on the subject seem to me to be irrelevant at all. It is very clear that this is seen as a rabbinical prohibition because we have not been able to find a good reason to prohibit it because of some master craftsman, so they say it is a rabbinical prohibition without explaining what makes it a prohibition that the sages established to be prohibited (two thousand years ago?). The prohibition because of a child also seems somewhat narrow to me, and the desire to prohibit it at all costs is clearly evident. Likewise, the prohibition because of the ovadeen dechul also does not fall into this category of the basket of prohibitions that have no real basis. After all, it is no different from so many things that we do on Shabbat that seem to be ovadeen dechul like a Shabbat clock. (The category of ovadeen dechul is empty of content because whatever you want to prohibit, you put into it and whatever you don't want, you take out). Only the prohibition based on custom can convince, but this is a custom that, in my opinion, was created without reason and based on a fundamental error, and since there is no basis for its prohibition, it is not a weak halakha that I should follow the custom.
I know I've barged into an open door. Still, I want to ask two questions. Was the rabbi convinced for one reason to forbid it, and why? And second, if I'm not convinced, can I trust my opinion of the law? thanks