When Telegram first introduced public chats in March 14, 2016, Iranians and some other countries were banned to join public chats. Telegram is the most popular IM in Iran so there are many spammers from Iran.

Recently Telegram removed this restriction. Iranian users can now join to public chats but In read_only mode,
Which means they can only read the messages of the group, but they can't send any messages.
Iranian users can create public chats with their Iranian numbers, join to the public chats which have Iranian owner and send messages there.

Please pay attention, This restriction is from Telegram only. The admins of the public chats have nothing to do with that and they can not solve this problem.

t.me/plusmsgrir/59