Have you tried checking your call settings to see if unknown/private numbers are blocked? If the company is trying to call you on a private number, it could be blocked due to this. You can check this option by heading into the Phone app > 3 dots in the top right > Settings > Block numbers > Switch off the 'Block unknown/private numbers'.
@Stevefl: If you head back to the Phone app > Tap the 3 dots in the top right > Settings > Block numbers, are any of the affected numbers in the block list at the bottom of the screen. If not, tap on 'Block spam and scam calls' and toggle this option off temporarily to see if this is a contributing factor.