docs(phone-numbers): free Vapi numbers cannot be ported out

[chiranjeet-vapi]

Summary

• Free Vapi numbers are provisioned on Vapi's own carrier accounts, so there's no customer-facing account number or port-out PIN to hand out and the number cannot be ported to another carrier.
• Adds a restriction bullet and an FAQ entry to fern/phone-numbers/free-telephony.mdx so support can point customers here instead of writing this up ad hoc each time.

(Replaces #1264, which carried an already-merged Krisp commit from a stale base branch.)

Test plan

• Fern preview renders the new bullet and accordion correctly

[lightsage-app]

Lightsage docs evals

Waiting for the staging docs URL before running evals.

Lightsage will start the selected PR evals automatically when GitHub reports a successful docs deployment for this PR. This usually happens within 15 minutes.

Commit: 1af53b7
Status: waiting for staging docs URL