Operating system name and version: CENTOS 7
Version of smsd:
Smsd installed from: sources / package repository / from elsewhere...
Name and model of a modem / phone: Huawei E169 & Huawei K3520
Interface: USB

Everything was fine until today! Everyday I schedule server to send me an SMS all morning at 09:00. Today everything was fine.
At the end of the morning i tried a auto-respond SMS from playsms. That was when i suspect something wasn't rigth! Sms would get to my phone!

I've sended SMS, system did work. But sending sms, NO! I'v took of SIM card and inserted on cellphone! Messages in & out with no problem.
Inserted again SIM card on Huawei K3520. Calls are on going, receive SMS works too. But sending SMS, NO!

It's allways giving 500 Error.

As I have two modems working, I tried to change USB port to Huawei E169!
All messages were received. But could send an SMS. Again Error 500

Here is some log:
2021-02-18 19:45:20,5, smsd: Moved file /var/spool/sms/outgoing/out.20210218194518.0.1.168 to /var/spool/sms/checked
2021-02-18 19:45:53,3, GSM91: The modem answer was not OK: +CMS ERROR: 500 (Unknown error)
2021-02-18 19:45:53,5, GSM91: Waiting 4 sec. before retrying
2021-02-18 19:46:29,3, GSM91: The modem answer was not OK: +CMS ERROR: 500 (Unknown error)
2021-02-18 19:46:29,5, GSM91: Waiting 4 sec. before retrying

Here is a fresh Log os smsd.conf:
2021-02-18 19:56:59,6, GSM91: Checking if modem needs PIN
2021-02-18 19:56:59,6, GSM91: Initializing modem
2021-02-18 19:57:00,6, GSM91: Signal Strength Indicator: (19,99) -75 dBm (Good), Bit Error Rate: not known or not detectable
2021-02-18 19:57:00,6, GSM91: Checking if Modem is registered to the network
2021-02-18 19:57:01,6, GSM91: Modem is registered to the network
2021-02-18 19:57:01,6, GSM91: Location area code: 0C, Cell ID: 4FAE
2021-02-18 19:57:01,6, GSM91: Signal Strength Indicator: (19,99) -75 dBm (Good), Bit Error Rate: not known or not detectable
2021-02-18 19:57:01,6, GSM91: Selecting PDU mode
2021-02-18 19:57:03,5, GSM91: CGSN: 353xxxxxxxxxxxx
2021-02-18 19:57:03,5, GSM91: IMSI: 268xxxxxxxxxxxxx
2021-02-18 19:57:03,6, GSM91: Checking if reading of messages is supported
2021-02-18 19:57:03,7, GSM91: ## Start of device details
2021-02-18 19:57:03,7, GSM91: # Manufacturer identification:
2021-02-18 19:57:04,7, GSM91: -> AT+CGMI
2021-02-18 19:57:04,7, GSM91: <- huawei
OK
2021-02-18 19:57:04,7, GSM91: # Model identification:
2021-02-18 19:57:04,7, GSM91: -> AT+CGMM
2021-02-18 19:57:04,7, GSM91: <- K3520
OK
2021-02-18 19:57:04,7, GSM91: # Revision identification:
2021-02-18 19:57:04,7, GSM91: -> AT+CGMR
2021-02-18 19:57:05,7, GSM91: <- 11.314.12.00.00
OK
2021-02-18 19:57:05,7, GSM91: # New message indications, list of supported modes:
2021-02-18 19:57:05,7, GSM91: -> AT+CNMI=?
2021-02-18 19:57:05,7, GSM91: <- +CNMI: (0,1,2),(0,1,2,3),(0,2),(0,1,2),(0,1)
OK
2021-02-18 19:57:05,7, GSM91: # New message indications, current settings:
2021-02-18 19:57:05,7, GSM91: -> AT+CNMI?
2021-02-18 19:57:06,7, GSM91: <- +CNMI: 2,0,0,2,1
OK
2021-02-18 19:57:06,7, GSM91: # Preferred message storage, list of supported mem's:
2021-02-18 19:57:06,7, GSM91: -> AT+CPMS=?
2021-02-18 19:57:06,7, GSM91: <- +CPMS: ("ME","MT","SM","SR"),("ME","MT","SM","SR"),("ME","MT","SM","SR")
OK
2021-02-18 19:57:06,7, GSM91: # Phonebook storage, available mem's:
2021-02-18 19:57:06,7, GSM91: -> AT+CPBS=?
2021-02-18 19:57:07,7, GSM91: <- +CPBS: ("SM","DC","FD","LD","MC","ME","RC","EN","ON")
OK
2021-02-18 19:57:07,7, GSM91: # List messages, list of supported stat's:
2021-02-18 19:57:07,7, GSM91: -> AT+CMGL=?
2021-02-18 19:57:07,7, GSM91: <- +CMGL: (0-4)
OK
2021-02-18 19:57:07,7, GSM91: # Delete message, list of supported values:
2021-02-18 19:57:07,7, GSM91: -> AT+CMGD=?
2021-02-18 19:57:08,7, GSM91: <- +CMGD: (),(0-4)
OK
2021-02-18 19:57:08,7, GSM91: # Phone activity status, list of supported stat's:
2021-02-18 19:57:08,7, GSM91: -> AT+CPAS=?
2021-02-18 19:57:08,7, GSM91: <- +CPAS: (0,3,4)
OK
2021-02-18 19:57:08,7, GSM91: # TE character set, list of supported charset's:
2021-02-18 19:57:08,7, GSM91: -> AT+CSCS=?
2021-02-18 19:57:09,7, GSM91: <- +CSCS: ("IRA","GSM","UCS2")
OK
2021-02-18 19:57:09,7, GSM91: # TE character set, current setting:
2021-02-18 19:57:09,7, GSM91: -> AT+CSCS?
2021-02-18 19:57:09,7, GSM91: <- +CSCS: "IRA"
OK
2021-02-18 19:57:09,7, GSM91: ## End of device details
2021-02-18 19:57:09,6, GSM91: Checking memory size
2021-02-18 19:57:10,6, GSM91: Used memory is 0 of 30
2021-02-18 19:57:10,6, GSM91: No SMS received
2021-02-18 19:57:10,6, GSM91: Sending SMS from to 35193xxxxxxx
2021-02-18 19:57:10,6, GSM91: Checking if modem is ready
2021-02-18 19:57:10,6, GSM91: Pre-initializing modem
2021-02-18 19:57:11,6, GSM91: Initializing modem
2021-02-18 19:57:12,6, GSM91: Signal Strength Indicator: (19,99) -75 dBm (Good), Bit Error Rate: not known or not detectable
2021-02-18 19:57:12,6, GSM91: Checking if Modem is registered to the network
2021-02-18 19:57:12,6, GSM91: Modem is registered to the network
2021-02-18 19:57:12,6, GSM91: Selecting PDU mode
2021-02-18 19:57:36,3, smsd: FATAL ERROR: Looks like another smsd (27873) is running. I (26377) quit now.
2021-02-18 19:57:36,2, smsd: Smsd mainprocess terminated abnormally. PID: 26377.
2021-02-18 19:57:40,3, GSM91: The modem answer was not OK: +CMS ERROR: 500 (Unknown error)
2021-02-18 19:57:40,5, GSM91: Waiting 4 sec. before retrying
2021-02-18 19:57:44,6, GSM91: Checking if modem is ready
2021-02-18 19:57:45,6, GSM91: Pre-initializing modem
2021-02-18 19:57:47,6, GSM91: Initializing modem
2021-02-18 19:57:48,6, GSM91: Signal Strength Indicator: (19,99) -75 dBm (Good), Bit Error Rate: not known or not detectable
2021-02-18 19:57:48,6, GSM91: Checking if Modem is registered to the network
2021-02-18 19:57:49,6, GSM91: Modem is registered to the network
2021-02-18 19:57:49,6, GSM91: Selecting PDU mode
2021-02-18 19:57:58,5, smsd: Moved file /var/spool/sms/outgoing/send_THzPer to /var/spool/sms/checked
2021-02-18 19:58:20,3, GSM91: The modem answer was not OK: +CMS ERROR: 500 (Unknown error)
2021-02-18 19:58:20,5, GSM91: Waiting 4 sec. before retrying
2021-02-18 19:58:24,6, GSM91: Checking if modem is ready
2021-02-18 19:58:30,6, GSM91: Pre-initializing modem
2021-02-18 19:58:30,6, GSM91: Initializing modem
2021-02-18 19:58:34,6, GSM91: Signal Strength Indicator: (19,99) -75 dBm (Good), Bit Error Rate: not known or not detectable
2021-02-18 19:58:34,6, GSM91: Checking if Modem is registered to the network
2021-02-18 19:58:34,6, GSM91: Modem is registered to the network
2021-02-18 19:58:34,6, GSM91: Cell ID changed: 4FAE -> 0
2021-02-18 19:58:35,6, GSM91: Signal Strength Indicator: (19,99) -75 dBm (Good), Bit Error Rate: not known or not detectable
2021-02-18 19:58:35,6, GSM91: Selecting PDU mode
2021-02-18 19:59:02,3, GSM91: The modem answer was not OK: +CMS ERROR: 500 (Unknown error)
2021-02-18 19:59:02,4, GSM91: Sending SMS to 35193xxxxxxx failed, trying time 112 sec. Retries: 2.
2021-02-18 19:59:02,6, GSM91: Moved file /var/spool/sms/checked/send_VjuI3c to /var/spool/sms/failed/send_VjuI3c
2021-02-18 19:59:02,6, GSM91: Checking device for incoming SMS
2021-02-18 19:59:02,6, GSM91: Checking if modem is ready

An update:

PlaySMS can send every day, an sheduled test message, as I programed!
But sending an manual sms through "compose message", doesn't send! It gives Error 500!

If I try smssend command, it does the same error 500!

SOLVED

Issue: International Prefixes

Well, i'ved upload the csv file with no international prefixes! So, wente to smsd.conf and added the international prefixes on the file +351!

Everything was going well with no problem! But when I started the Play SMS service with subscribe sms, the system could't send, but could reeive. Why? Because, when the system was sending, there was an conflict with two international prefixes. So, the system could not send the sms with +351 and another +351!

I've re-uploaded the phonebook with international prefixes on each contact ad everything works correctly!