Old/Unused Number showing in suggestion when typing in phone number (input) field

If you encounter any challenges with third-party applications, the best course of action is to directly contact the app developer for further assistance. Here's a brief guide on how to do it:

1. Identify the app developer.
2. Visit their website or app store listing.
3. Look for their contact information or support channels.
4. Clearly describe the issue you are facing.
5. Provide any necessary information they may need.
6. Follow their instructions and provide feedback.
7. Be patient and follow up if needed.

By reaching out to the app developer, you increase your chances of receiving accurate and personalized support for the challenges you're experiencing. To contact an App developer --> How to contact an app developer - Apple Support

There isn't a direct way to remove individual suggestions from the iPhone keyboard for phone numbers. However, there are a few things you can try to get rid of those old SIM numbers:

• You can reset the keyboard dictionary, which will remove all custom words and data the keyboard has learned. To do this, go to Settings > General > Reset > Reset Keyboard Dictionary.
• Check your Contacts app to ensure that the old phone numbers associated with your SIM card are not stored in your contacts. If they are, delete or update the contact information accordingly.
• If you use iCloud to sync contacts across devices, ensure that the old phone numbers are not stored in your iCloud contacts. If they are, remove them from iCloud contacts.
• Over time, as you type new phone numbers more frequently, the old ones should become less likely to appear as suggestions.

Siri Learns from where you go and from what you have on your device.

Restore from a backup to make Siri unlearn. Else...

What I have experienced is when you keep using the iPhone regularly the contacts with whom you interact or places you go recently and regularly will are learnt by Siri.

If you do not interact with contact or stop going to a place then it will disappear automatically.

However, you may....

• To turn off Siri Suggestions, go to Settings  > Siri & Search, then turn off any of the following:

1. Suggestions in Search
2. Suggestions in Lookup
3. Suggestions on Lock Screen

For a specific app, tap the app, then turn off Show Siri Suggestions.

Your personal information—which is encrypted and remains private—stays up to date across all your devices where you’re signed in with the same Apple ID. As Siri learns about you on one device, your experience with Siri is improved on your other devices.

• Siri is designed to protect your information, and you can choose what you share. To learn more, go to Settings  > Siri & Search > About Search Suggestions & Privacy.

You may get better and prompt responses if you may kindly provide the iPhone Model that you have and the iOS version if it is known to you and re-post.

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.