Failed to send notification using device/server

Hi all !

I have fully configured SCOM 2012sp1 server with directly attached GSM-modem.I already followed this articlehttp://www.pavleck.net/2009/04/configuring-sms-in-opsmgr/ and configured baud rate 9600.Nofification channels and subscribtions are configured also well. Generally, everything works fine: SMS, Email notification. 

But sometimes (randomly) SCOM fails to send notification through SMS channel. There is alerts like these:

Failed to send notification using device/server 'MultiTech Systems USB GSM_GPRS Wireless Modem #2' over 'SMS' protocol to '+7xxxxxxxxxx'.Exception 'Microsoft.EnterpriseManagement.HealthService.Modules.Notification.SmsNotificationException': Sms error 'Send message failed' (16012)

or

Is it problem with modem/drivers ? How can i troubleshot it?

Thanks for yours reply.