How SS7 attack first enter into SS7 network?

SS7 attacks can't be done via ISDN lines.

In order to attack the SS7 network, the attacker has to be "on the SS7" network. The SS7 network connects telco companies together, but it is not extended to clients. What this means is that the attacker is either has access to the network administrator at a telco company, or the attacker is the network admin at the telco company.

Luckily, SS7 attacks can't be done by default by skiddies. But countries where the admins are not payed very well will pose a significant threat to the whole world. Or countries where the intelligence agency is interested in attacking the another country is also a huge threat.