Acknowledging receipt of data sent by isolated volunteers

I have been using ODK for a few years, but I have only recently joined the ODK forum, and this is my first question! I had a good look to see if there was a similar discussion and couldn't find one.

We use ODK to collect data about supported students in 5 countries in sub Saharan Africa, all in a rural context. Designated teachers in partner schools have received training to use ODK on phones or tablets. We use Jitterbit to map data from ODK into our Salesforce database. We used to pay for a SMS reply system so that when users send data that reaches our Salesforce database, a SMS reply 'pings' back acknowledging receipt and thanking the volunteer for sending the data.

We no longer use this, partly because it's expensive, and partly because the original SIM cards from the phones are usually switched, so we don't have the information required to send these SMS. The concern is that isolated volunteers don't get an automated reassurance that their data was sent. This results in duplicate records being sent, lack of interest and motivation.

A couple of our countries use the ODK Collect app v 1.12 and above, so I have suggested that we train users to check "View Sent Form", as well as build the capacity of local staff to interact with isolated volunteers, and reassure them that they have indeed sent the data.

However, there is still demand for something like our old SMS reply system. I wondered if other ODK app users had experienced anything similar, (especially if working with large numbers of volunteers who don't return to a central location very often)? I would appreciate any ideas or suggestions!
Thank you!