New usage docs

Technically, if it's not a mobile phone you're calling, it's not a MSISDN, so maybe we should give it another name. Remember that you have three numbers in case of forwarding calls: your number, the caller, and the number the call is forwarded to. How will we represent this?

How will we handle multi-legged roaming calls? In the CDRs we have one leg with the "actual" number, and one with the country you're calling to or country you're receiving the call.