Here are some useful resources
General
http://en.wikipedia.org/wiki/Caller_ID
Technical
http://melabs.com/resources/callerid.htm
Modems
http://www.ainslie.org.uk/callerid/cli_faq.htm#Q_18
Transmission at 1200bps makes it easy to read from a PICAXE. Fortunately the data stream has a fixed-length format that will make it easy to decode (from Ref 2)
"Here is an example of a SDMF message. Each byte is in HEX.
04 12 30 39 33 30 31 32 32 34 36 30 39 35 35 35 31 32 31 32 51
04 - message type word - 4 indicates SDMF 12 - 18 decimal, number of bytes in date, time and phone number 30,39 - 09, September (ASCII) 33,30 - 30, 30th day (ASCII) 31,32 - 12, 12 hour (ASCII) 32,34 - 24, 24 minutes (12:24 PM) (ASCII) 36,30,39 - 609, Area code (ASCII) 35,35,35 - 555, prefix (ASCII) 31,32,31,32 - 1212, sufix (ASCII) 51h = Checksum Word
Thus, the CID string can be summarized as follows:
The message is in SDMF format, consisting of 18 bytes of information, not including the checksum. The call was made on September 30 at 12:24pm. The calling party's phone number was (609)555-1212."
Yes . . . I did spend my working life with a Telco.
Mike