To use Blink you need a SIP account from a SIP service provider. Similar to using an email program, you must configure Blink with the account you have obtained from your service provider. If things do not work the way you expect, there can be several reasons.
The cause may be located by Blink software itself, the network connectivity or the SIP Service Provider. While Blink may not be bug free, based on experience so far most of the problems are related to the network connectivity (packet loss, presence of a broken SIP ALG router) or/and the SIP Service Provider (most providers only support Voice calls). It is not the end-user who should understand the mechanics about how things work, so the main responsibility of making sure that things work correct falls into the hands of the SIP service provider who must properly setup its SIP infrastructure.
To eliminate some possible causes, try to use the built-in accounts provided by Blink, all features are known to be working using these accounts.