Why didn't my customer receive the text message?

Whilst we are accustomed to messages arriving within a few minutes, SMS isn't an instant messaging service. For example, even within network, Orange UK guarantees delivery within a certain time, and that time is 24 hours.

Location Issues

The location of the sender or receiver can create a delay in text message delivery. What causes this varies from handset to handset, but common reasons are being on the border of two cell coverage areas or travelling at speeds above about 50km/h. This is most likely happening if messages arrive all at once. If a mobile device is located outside a network's coverage area or in a spot where the network signal is blocked such as in a mountainous region, a delay in transmission may occur. In urban areas, tall buildings can also cause transmission issues. Being in an old building with poor cell reception is another possible culprit.

Mobile Device Issues

Problems with a mobile device may cause delays in text message transmission. The most obvious cause is a device that has been turned off, but a weak or uncharged battery may also negatively impact message delivery. Devices that have an adjustable antenna may experience transmission difficulties if the antenna has sustained damage or has not been fully raised.

Different Networks

A text sender and receiver using different networks may have a greater chance of experiencing texting delays than those using the same network, because of communication between networks or the carrier prioritizing their own traffic.

Network Traffic

Texting during periods of heavy network use impacts text delivery speed. Periods of increased traffic may create congestion on the local network and delay arrival of messages to the handset.

Information from: https://help.nexmo.com/hc/en-us/articles/204014893-What-causes-a-delay-in-delivering-sms-