We currently have SMS notifications set up by way of a Business Rule for a particular technician group when a high priority incident is created in their group. The SMS notifications are currently sending via mobile service provider (not an SMS gateway provider); all the phones are with Verizon, so the SMS mail IDs are configured with @vzwpix.com on the end of the phone number. Probably about 70% of the time, the notifications come through as expected. The other 30%, the notifications will arrive hours or even days late. The latest issue was a high priority incident that came in on Thursday, and the technicians were all simultaneously alerted by SMS on Saturday at 3am. 

We currently have no problem with our mail server settings in ManageEngine, which fetch and send emails all day long seamlessly (and which I assume the SMS emails-to-text are using as well, so it doesn't seem to be mail server related.)

Any idea what is causing this sporadic lag?