I think you are referring to the prepaid sim market where users go to shops, and ask shop owners to recharge for low amounts. The conflict arises when the shop owner writes a different number in his notebook or keys in a wrong number into his mobile while doing a recharge for the customer . In the rarest of occasions, there would be some server problem as a result of which even if he keys in the correct number, the customer does not get his recharge on the spot, often leading to frustration.
Have you tried discussing this idea with a trusted shopowner, who can fill you in on the details of how they make their income via prepaid sim recharge ? They can shed more light on this topic, and any other pain points they face. Try talking to more than one person to get a better idea.
Coming to the low cost device, the kernel of your idea is the connectivity protocol to the telecom service provider network. i googled and found https://www.rechargemy.com/ which gives you a load of info on this topic.
An idea for a custom device to process recharge flies in the face of existing innovations like smartphone apps . The whole idea is to make the procedure seamless for the shopowner, the telecom company and the customer . By introducing a new hardware device into this situation, i think you would be making the system more complicated than it already is , but thats just my opinion.
Having said that there IS a huge market around payments and recharge in general, which is what BHIM, AIRTEL Payments, and PayTM and all the other mobile wallets are hotly contesting.
In order to fix the specific issue that manual error occuring as part of the interaction b/w customer and shopowner, i think a mobile application (pilot an android ) is a more sensible way to go about it.In fact, seems like its already been done.
Reading your other posts on startups forum , i sense you are keenly interested in building a business related to electronic equipment manufacture, and we can maybe take that discussion offline, i'd like to hear more about your vision before suggesting anything.
All the best!