Messages sent to a specific contact are being delivered to their old phone number

Re: Messages sent to a specific contact are being delivered to their old phone number

You might want to check:

whether the phone number you have listed for that contact in the Contacts app is correct.

If needed you can: Edit contacts on iPhone

Apple Suppoert's article:Edit contacts on iPhone - Apple Support

Eg: says:

1. Go to the Contacts app on your iPhone.
2. Tap a contact, then tap Edit.
3. Enter or update the contact’s information.
4. When you’re finished, tap Done.

On models with Dual SIM,

when you call or text a contact, iPhone uses the same line you used for your previous communication with this contact by default. To choose a preferred line for phone calls and SMS/MMS message conversations, select

the contact, tap default (below the contact’s name), then choose a line.

To change how your contacts are sorted and displayed, go to Settings  > Apps > Contacts."