A customer says they never got the message
Check the delivery record on the job first. It is usually a wrong digit, a full inbox, or a network that dropped it.
Every message has a delivery record, so this is answerable rather than arguable.
Check the job
Open the job and look at Messages. Each one shows sent, delivered or failed, with a time. Delivered means the network confirmed it reached the handset.
If it says failed
Almost always a wrong number. Fix it on the customer record and press Send again.
If it says delivered
Then it arrived. Common reasons it was not seen: it went to a second phone, the number belongs to a partner, or the customer has messages from unknown numbers filtered. Send them the job link by email or read it to them; both work.
If it says sent but never delivered
The network accepted it and never confirmed. This is rare and usually resolves within an hour. If it does not, send again — a duplicate is better than a missed collection.