Hey there RodvS,
Thanks for reaching out to the Apple Support Communities.
To get started, you stated that the two contacts who's calls are going to voicemail are listed in your Contacts app, correct? If so, does the issue persist after restarting your iPhone? While restarting may seem like a simple step, it can sometimes help with resolving some unexpected issues like this: Restart your iPhone - Apple Support
How to restart your iPhone X, 11, 12, 13, or later
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
If the issue persists after restarting your iPhone, and if you go into Settings > Phone > Silence Unknown Callers, is this setting enabled here?
If it is, while Silence Unknown Callers should only cause callers not in your Contacts app to be rerouted to voicemail, try turning this setting off and test the issue again. More information on this feature can be found here: Detect and block spam phone calls - Apple Support
If Silence Unknown Callers is already disabled, or if the issue persists after disabling this setting, do you by chance use any Focus features on your iPhone such as Do Not Disturb? If you do, depending on how you have the Focus settings configured, it could cause certain contacts to go straight to voicemail while the Focus is enabled. For more details on using Focus features on your iPhone, check out this next link here: Use Focus on your iPhone or iPad - Apple Support
Keep us updated on what you find!
Take care.