thanks for the hint. I didn't know an ANT slave always searches for the master first. But after reading the document mentoined by you and additionally "AN11 ANT Channel Search and Background Scanning Channel", my problem was clear - I have multiple search channels open simultaneously. Thanks again for the hint, that helped a lot sorting out my problem.
Current SE phones like Xperia X8 have no capability for extended messaging and thus no background search channel can be opened. Are there any current mobile phones that support a background search or will there be any devices in the (near) future?