Out of sheer desperation I decided to RTFM and found this snippet....
Text: content ahead (0…9,A…F), if dcs(by AT+CSMP)is a 7bit ASCII
character, its length range is 0-160; if dcs is a 8bit ASCII character, or its
length range is 0-140; if dcs is a UCS2 character, or its length range is 0-140.
I think that this means that if I’m using 7 bit ASCII characters then a 160 character message should be possible. Since I'm getting an error above 140 characters I guess I must be working with 8bit ascii or UCS-2 characters.
Reading further it seems I can change the character set using AT+CSMP. Page 67 shows the options....
Coding description of <dcs> information with a default value of 0.
0 Default alphabet
1 8 bit data
I’ve tried all 3 options, none of them seem to make any difference to the error I’m getting.
Any suggestions? I can't be the only one using this module to send messages more than 140 characters?