Contrary to popular belief, sign language is not a universal language. Like spoken languages, sign languages around the world are different.
International Sign Language (ISL), formerly known as "Gestuno", is an artificially devised sign language used at Deaf international events and meetings such as World Congress of the Deaf run by World Federation of the Deaf (WFD), Deaflympics run by CISS/ICSD, and such.
Gestuno is to Deaf as Esperanto is to hearing.
International Sign is composed of vocabulary signs from different sign languages that Deaf people agreed to use at international events and meetings.
ISL has been developed by Deaf people at the international events, such as World Congress of the WFD and World Games for the Deaf. The signs were selectively loaned from different sign languages around the world.
For example, look at these three words for "government" in three different sign languages.
Most likely a native or fluent signer would choose the third one which is more iconic (or "onomatopoeic") with the 'crown' and its association with the Scandinavian governments.
First of all, sign language is not iconic per se; however, words are selected that might be most possibly understood. Sign languages around the world are abstract and incomprehensible to one another (foreign signers).
International organization Deaf officials are competent in International Sign to communicate with one another at meetings and official functions. Experienced Deaf world travellers and globe-trotters also use ISL to communicate with other Deaf travelers and sometimes locals. Not all deaf travelers and locals know ISL.
However, International Sign is never practiced in home lands. Like Esperanto, it's not as productive as their natural languages. Native sign languages are used in their home countries.