With the sipgate API you, or your program, can make calls to the API at anytime. This allows to initiate calls, retrieve balances and much more.
With sipgate.io the request comes from the other direction, namely the sipgate.io Servers. The timing is not arbitrary, but rather whenever an event occurs (for example, a new phone call). sipgate.io is therefore a "PUSH-API", because it 'pushes' the event to you.
The sipgate API is free.
sipgate.io PUSH-API is available as an optional feature within your sipgate team account for a monthly fee of £29.95 GBP.
The functions of the two APIs are complementary and may be used together.