If my call volume doubles, does my price double?

No, the plan isn't indexed to call count: $149/month covers a line's normal volume, peaks included. What triggers a tier change is the number of lines, not activity: a single line getting 50 missed calls a week costs the same as one getting ten. In case of genuinely exceptional usage, you see consumption and unit price in the dashboard before any billing — never a delayed surprise. It's at multi-line, from $499/month, that pricing follows your business structure, not the volume of one extra caller.