What "25 lines" actually means: channels vs users

It is worth being clear about what the twenty-five refers to. The account provides twenty-five concurrent SIP channels — that is, up to twenty-five calls happening at the same time. It is not a cap on how many people, extensions or handsets you have.

You can register far more than twenty-five devices and create as many extensions as you like; the limit only applies to simultaneous conversations. In practice a team of forty or fifty people rarely has more than twenty-five on the phone at once, so twenty-five channels comfortably covers a busy office or support desk. Because there are no per-seat or per-extension charges, adding more people costs nothing — you are only ever paying the single flat monthly fee for the channels.