I have a conflict of interest here (I am an advisor to Cape, also a security expert, and my company has done security audits for Cape), you should absolutely look more deeply into what Cape has created. Their service is fundamentally different than other "security-focused cell providers" (mostly snake oil IMHO) because Cape wrote their own mobile core, nearly from scratch. They control the whole software stack and have done really innovative things with it.

Here are a few things you might want to look at more closely:

Encrypted voicemail uses public key crypto: https://www.cape.co/blog/product-feature-encrypted-voicemail

How they use full control of the mobile core to detect SS7 signaling attacks https://www.cape.co/blog/product-feature-network-lock

Swapping SIMs is done via digital signatures, not customer support https://www.cape.co/blog/cape-product-feature-secure-authent...

They're the only provider that can rotate your IMSI, and do it continuously for you https://www.cape.co/blog/product-feature-identifier-rotation

They're also one of very few organizations doing original research on cell network security:

Collaborating with the EFF to release software for detecting cell site simulators (e.g, imsi catchers et al) https://www.cape.co/blog/how-eff-and-cape-collaborated-to-im...

Identifying novel weaknesses for physically tracking people on cell networks https://dl.acm.org/doi/pdf/10.1145/3636534.3690709