If you can receive text messages from these users, then self-evidently Tesco Mobile have routing in-place for your number prefix - the first 5 digits of your number.
Have these users been able to call you in the past? Have you recently changed your number? Out of curiosity, do you have a ported-in number or one issued by EE? Are you able to receive calls from users on other networks, including O2?
The end-game here would be for your contacts to log a "cannot call number" fault with their provider. Evidence that text messages are sent & received to demonstrate that "there is no such number" is nonsense. If they have been able to call you in the past, locate dates & times of these calls if possible. Tesco may ultimately need to instigate a call-trace to establish the reason for failing calls, although I would not expect all their frontline CS staff to be familiar with this process.
Given that Tesco is a budget MVNO and may need to internally liaise with O2 to pursue this, your contacts may need persistence.
