The ChatSecure Messenger App is a free, open source XMPP/Jabber chat-client for Apple iOS.
At first, the own Jabber-ID needs to be entered in ChatSecure:
Contrary to popular messengers, the chat partners in XMPP/Jabber are not automatically available. They need to be added once.
Normally, an indication that says “Pending Approval” will appear after tapping “Write Message” again. It will remain there until the chat partner has accepted the request for communication. When this happens, you will receive a message saying “You and your chat partner are now friends.”