Baseband concerns are legitimate. A good tinfoil hat approach is to use an iPod touch running an end-to-end encrypted messaging/calling app of your choice, connected to a secure hotspot. Cuts out most baseband vulnerabilities (since your data is encrypted before touching any hardware or software connected to a potentially compromised baseband).
All other concerns raised elsewhere here still apply, but the baseband threat is mitigated. Worth it...? Check that threat model again.
