Operating system name and version: Centos 7
Version of smsd: 3.1.21
Smsd installed from: sources
Name and model of a modem / phone: Nokia 22 PBX GSM Gateway
Interface: Nokia 22 PBX GSM Gateway

Smstools3 cannot recognize answer from my gsm gateway. With check_network=0 works good. Can you help?

2017-09-08 12:37:05,2, smsd: Smsd mainprocess terminated. PID: 29607, was started 17-09-08 12:32:58, up 4 min.
2017-09-08 12:37:06,2, smsd: Smsd v3.1.21 started.
2017-09-08 12:37:06,2, smsd: Running as smstools:smstools (976:96.
2017-09-08 12:37:06,7, smsd: Running startup_check (shell): /var/spool/sms/incoming/smsd_script.S1yoDj /tmp/smsd_data.y9zabE
2017-09-08 12:37:06,7, smsd: Done: startup_check (shell), execution time 0 sec., status: 0 (0)
2017-09-08 12:37:06,4, smsd: File mode creation mask: 022 (0644, rw-r--r--).
2017-09-08 12:37:06,5, smsd: Outgoing file checker has started. PID: 29799.
2017-09-08 12:37:06,5, GSM0: Modem handler 0 has started. PID: 29801. Will only send messages.
2017-09-08 12:37:06,7, smsd: All PID's: 29799,29801
2017-09-08 12:37:06,6, GSM0: Checking if modem is ready
2017-09-08 12:37:06,7, GSM0: -> AT
2017-09-08 12:37:06,7, GSM0: time 0 ms Command is sent, waiting for the answer. (5)
2017-09-08 12:37:11,7, GSM0: No answer, put_command expected (OK)|(ERROR)|(0)|(4), timeout occurred. 1.
2017-09-08 12:37:11,7, GSM0: time 4337 ms <-
2017-09-08 12:37:11,7, GSM0: -> 
2017-09-08 12:37:11,7, GSM0: time 0 ms Command is sent, waiting for the answer. (5)
2017-09-08 12:37:16,7, GSM0: No answer, put_command expected (OK)|(ERROR)|(0)|(4), timeout occurred. 2.
2017-09-08 12:37:16,7, GSM0: time 4878 ms <-
2017-09-08 12:37:16,7, GSM0: -> AT
2017-09-08 12:37:16,7, GSM0: time 0 ms Command is sent, waiting for the answer. (5)
2017-09-08 12:37:16,2, GSM0: read_from_modem: count=0, got=6: 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:16,7, GSM0: time 40 ms <- OK
2017-09-08 12:37:16,6, GSM0: Pre-initializing modem
2017-09-08 12:37:16,7, GSM0: -> ATE0
2017-09-08 12:37:16,7, GSM0: time 0 ms Command is sent, waiting for the answer. (10)
2017-09-08 12:37:16,2, GSM0: read_from_modem: count=0, got=6: 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:16,7, GSM0: time 20 ms <- OK
2017-09-08 12:37:16,7, GSM0: -> AT+CMEE=1;+CREG=2
2017-09-08 12:37:16,7, GSM0: time 0 ms Command is sent, waiting for the answer. (10)
2017-09-08 12:37:16,2, GSM0: read_from_modem: count=0, got=6: 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:16,7, GSM0: time 20 ms <- OK
2017-09-08 12:37:16,6, GSM0: Checking if modem needs PIN
2017-09-08 12:37:16,7, GSM0: -> AT+CPIN?
2017-09-08 12:37:16,7, GSM0: time 0 ms Command is sent, waiting for the answer. (10)
2017-09-08 12:37:16,2, GSM0: read_from_modem: count=0, got=22: 0D[.] 0A[.] 2B[+] 43[C] 50[P] 49 4E[N] 3A[:] 20[ ] 52[R] 45[E] 41[A] 44[D] 59[Y] 0D[.] 0A[.] 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:16,7, GSM0: time 40 ms <- +CPIN: READY OK
2017-09-08 12:37:16,7, GSM0: -> AT+CSQ
2017-09-08 12:37:16,7, GSM0: time 0 ms Command is sent, waiting for the answer. (10)
2017-09-08 12:37:16,2, GSM0: read_from_modem: count=0, got=21: 0D[.] 0A[.] 2B[+] 43[C] 53 51[Q] 3A[:] 20[ ] 33[3] 31[1] 2C[,] 39[9] 39[9] 0D[.] 0A[.] 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:16,7, GSM0: time 40 ms <- +CSQ: 31,99 OK
2017-09-08 12:37:16,6, GSM0: Signal Strength Indicator: (31,99) -51 dBm (Excellent), Bit Error Rate: not known or not detectable
2017-09-08 12:37:16,6, GSM0: Checking if Modem is registered to the network
2017-09-08 12:37:16,7, GSM0: -> AT+CREG?
2017-09-08 12:37:16,7, GSM0: time 0 ms Command is sent, waiting for the answer. (10)
2017-09-08 12:37:16,2, GSM0: read_from_modem: count=0, got=33: 0D[.] 0A[.] 2B[+] 43[C] 52[R] 45[E] 47[G] 3A[:] 20[ ] 32[2] 2C[,] 2C[,] 22["] 38[8] 34[4] 44[D] 32[2] 22["] 2C[,] 22["] 30[0] 30[0] 45[E] 39[9] 22["] 0D[.] 0A[.] 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:16,7, GSM0: time 40 ms <- +CREG: 2,,"84D2","00E9" OK
2017-09-08 12:37:16,5, GSM0: MODEM IS NOT REGISTERED, WAITING 1 SEC. BEFORE RETRYING 1. TIME
2017-09-08 12:37:18,7, GSM0: -> AT+CSQ
2017-09-08 12:37:18,7, GSM0: time 0 ms Command is sent, waiting for the answer. (10)
2017-09-08 12:37:18,2, GSM0: read_from_modem: count=0, got=21: 0D[.] 0A[.] 2B[+] 43[C] 53 51[Q] 3A[:] 20[ ] 33[3] 31[1] 2C[,] 39[9] 39[9] 0D[.] 0A[.] 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:18,7, GSM0: time 40 ms <- +CSQ: 31,99 OK
2017-09-08 12:37:18,5, GSM0: Signal Strength Indicator: (31,99) -51 dBm (Excellent), Bit Error Rate: not known or not detectable
2017-09-08 12:37:18,7, GSM0: -> AT+CREG?
2017-09-08 12:37:18,7, GSM0: time 0 ms Command is sent, waiting for the answer. (10)
2017-09-08 12:37:18,2, GSM0: read_from_modem: count=0, got=33: 0D[.] 0A[.] 2B[+] 43[C] 52[R] 45[E] 47[G] 3A[:] 20[ ] 32[2] 2C[,] 2C[,] 22["] 38[8] 34[4] 44[D] 32[2] 22["] 2C[,] 22["] 30[0] 30[0] 45[E] 39[9] 22["] 0D[.] 0A[.] 0D[.] 0A[.] 4F[O] 4B[K] 0D[.] 0A[.]
2017-09-08 12:37:18,7, GSM0: time 40 ms <- +CREG: 2,,"84D2","00E9" OK
2017-09-08 12:37:18,5, GSM0: MODEM IS NOT REGISTERED, WAITING 1 SEC. BEFORE RETRYING 2. TIME
....

You device is missing status parameter and therefore checking for registration is not supported.

To the next version of smsd I could add a setting like check_network_ok, which in your case would be +CREG: 2,,. However, I need to see what is the answer in your case when the modem is NOT registered. If it cant tell anything about status of registration, disabling the whole checking is the best solution, as you already have done.