string type alphanumeric representation
of <da> or <oa>, corresponding to an entry
found in the phonebook; used character
set is the one selected with command
+CSCS
In case of Text mode, if there is a received message in location <index>,
the output has the following format (the information written in italics will
be present depending on +CSDH last setting):
+CMGR:<stat>,<oa>,<alpha>,<scts>
[,<tooa>,<fo>,<pid>,<dcs>,<sca>,<tosca>,<length>]
<CR><LF><data>
In case of Text mode, if there is either a sent or an unsent message in
location <index> the output format is:
+CMGR:<stat>,<da>,<alpha>
[,<toda>,<fo>,<pid>,<dcs>,[<vp>],<sca>,<tosca>,<length>]
<CR><LF><data>