Jakelson Is the phone number required for sign-up? Or is that the number you would be porting over or the new number you would be getting?
Here is their privacy policy (at the moment -- their web pages seem to be moving around): https://www.cape.co/privacy-summary
Beyond that, I haven't signed up with them, so I don't know exactly what they require for customers in which circumstances during the sign-up process.
My impression is that being a Cape subscriber means creating a data trail similar to being a customer of a regular cellular carrier, but the data is somewhat split across multiple parties. Their materials are very up-front about their cellular carrier collecting the standard cellular tracking information, for example, and they indicate Cape themselves keeps 60 days of location data (see the "Location" and "Call Detail Records" rows of the table). It appears that their payment processor, Stripe, collects standard credit-card information from customers, but Cape indicates that much of the information (name, address) remains with Stripe.
If the cellular carrier has the standard detailed location track for the device, and if Stripe has standard information about the payer, but neither the carrier nor Stripe has both the identity information and the location information, that may be an improvement -- in theory, breaches should be less damaging. Of course, the government can put the pieces together, and Cape's privacy policy does mention longer record retention during fraud investigations.