GET 1 subscriber.by-imsi-901991234567891.msisdn
GET_REPLY 1 subscriber.by-imsi-901991234567891.msisdn none

SET 2 subscriber.by-imsi-901991234567891.msisdn 555666
SET_REPLY 2 subscriber.by-imsi-901991234567891.msisdn OK

GET 3 subscriber.by-imsi-901991234567891.msisdn
GET_REPLY 3 subscriber.by-imsi-901991234567891.msisdn 555666