If you sign up and give your phone number are you then plagued by unwanted calls and texts?
You’ll have to check Google’s terms & conditions for that (good luck!). If you already have a Google account (for G-Mail or Google Drive for example), then you simply sign in with that – one account for all. I do not recall having to provide a phone number specifically when creating a Google account, I have since added a phone number for the purposes of account/password recovery. I have never been called up about anything as a result of signing up for a Google account – there are far more cost=effective and less intrusive means of directing “content of interest” your way.
The Google+ account settings themselves provide rather fine control over use of your information. I took a look at the phone contact settings, and notifications when “community activity” for groups you are signed up to seems to be the default, but I have only ever received e-mails for such.