I suppose it means the very first one in your life.

In this context you’re just putting more emphasis on the fact it’s your first phone, as you already mentioned at the beginning of the sentence.

“有生以来”的意思。

Hello, It means you have never had a phone before. This is the first. I hope this answers your question. If you’d like to practice speaking. I’d be happy to help you develop a study plan. Feel free to view my teaching profile and class offerings and send me a message if you think I can help you. Regards.

In this sentence it is an adverb that modifies the adjective "first". You could also write it like this: "I just got my first ever phone this September". "First ever" means the first one in your entire life. It's interesting, and perhaps confusing, that "ever first" and "first ever" do not mean the same thing at all. "Ever first" is much less common. I believe that "ever first" means "forever first", so it refers to the future, unlike "first ever" which refers to the past.