Most common reason for returning OK would be that there are no messages stored in memory.
To check if that's the case run the reading command without setting the Preferred Storage.
+CPMS: used_space1,max_space1,used_space2,max_space2,used_space3,max_space3
used_space1 is the number of messages currently stored in the message storage area specified by the message_storage1 parameter.
max_space1 is the maximum number of messages that can be stored in the message storage area specified by the message_storage1 parameter.
used_space2 is the number of messages currently stored in the message storage area specified by the message_storage2 parameter.
max_space2 is the maximum number of messages that can be stored in the message storage area specified by the message_storage2 parameter.
used_space3 is the number of messages currently stored in the message storage area specified by the message_storage3 parameter.
max_space3 is the maximum number of messages that can be stored in the message storage area specified by the message_storage3 parameter.