Usually from the radio club the operator is a member of. Or it is simply made up. A lot of guys on the "domestic" DX channels (27385, 27395, up through 27505 or so, all LSB, with 27385 and 27425 being the more popular ones) will simply use a numerical callsign.
So, for example, say a station from Italy is a member of the Alfa Delta DX group, and is member number 499. His 11-meter callsign would be 1AD499. Of course a lot of stations on 11m just make up a callsign and throw their country's prefix on the front of it.
I also recommend that you take a look at the Freebanding Frequency Chart Here:
http://www.mds975.co.uk/Content/cb_radio_15.html#chart_02Most export radios are either 3-band (26515-27855), 6-band - the most common (25615-28305 or 26065-28755), or 12-band (25165-30105 or, in the case of the Superstar 158EDX and several other radios, 24265-29205). There are several variants to this but 25615-28305 and 25615-30105 are the most common. Most CB radios sold in Europe today are "multi-norm", which means you can select which country's frequency allocation you want to use and the radio gives you access to those channels. For example, the UK technically has two distinct CB bands, the familiar 26965-27405 40-channel allocation, with AM/FM/SSB allowed, and the UK-specific 27601.25-27991.25 FM-only 40-channel allocation on top of that. Germany has 80 channels as well. 26965 to 27405 make up channels 1-40, and 41-80 are 26565-26955. So buying a European radio and setting it to a different country's "mode" gives you access to lots of channels. But, there's more...
Many of these radios are now including "RU mode" (RU for Russia) and RU mode means 25615-30105, both AM and FM mode. Get yourself a radio that supports SSB and put it in "RU mode" and you've got several hundred channels at your disposal. Who cares if some of them are part of the 10-meter ham band?
Here's some more logs (starting at 2100 UTC 10/21/2015)
25950 FM - STL
25990 FM - STL
26135 AM - Spanish language (I think, I can tell there's a carrier there...)
26205 USB - Spanish language
26225 USB - Spanish language, Dominican Republic mentioned
26365 AM - Spanish language, with elaborate roger beeps
26375 AM - Very weak, but there is voice activity here
26405 AM - Truckers (English language, US accents)
26475 LSB - Spanish language, strong signals, consistent peaks so S9+
26500 USB - Spanish language
26585 AM - Busy with powerful stations from Mexico/Latin America as usual
26595 AM - Spanish language, strong signals
26605 AM - Spanish language, S5-S7, music
26635 AM - Sounds like truckers, weak
26645 AM - Spanish language, S1-S2 weak signals, with heavy OTH radar QRM
26715 AM - Puerto Rico, very strong as usual
26735 AM - Truckers, Southern US accents, with OTH radar QRM
26945 AM - Truckers
26975 AM - CB channel 2 - Truckers
27015 AM - CB channel 5 - Heavy QRM/heterodyne from Latin America
27025 AM - CB channel 6 - Very strong signals (they call this channel "The Superbowl" for a reason

)
27035 AM - CB channel 7 - similar to 27015
27055 AM - CB channel 8 - Spanish language, heavy QRM
27265 AM - CB channel 26 - Southern US high power stations (see also 27285)
27285 AM - CB channel 28 - See above
27345 AM - CB channel 34 - Spanish language, S5 peaking to S9, with AC hum on some signals
27385 LSB - CB channel 38 - North American SSB calling freq, several stations calling CQ/working DX
27425 AM - Spanish language
27435 USB - Spanish language
27445 LSB - US stations, with AM carrier QRM fading in and out
27455 USB - Latin American SSB calling freq
27475 USB - Spanish language, with whistles and echo FX, some stations peaking at +30 over S9
27505 AM - Spanish language
27515 AM - Spanish language - YL taxicab dispatcher with roger beeps
27555 USB - International SSB calling freq
27655 AM - US truckers
27655 LSB - Spanish language
27690 LSB - Spanish language
27695 LSB - Spanish language
27775 AM - Spanish language - possibly taxicabs
27915 AM - Spanish language, weak