Q: my phone is not sending messages to some contacts. It says invalid number?

Hello there, cinclare.

The following Knowledge Base article offers up some great in-depth steps for troubleshooting issues with Messages:

iOS: Troubleshooting Messages

http://support.apple.com/kb/ts2755

This particular section offers some key pointers:

If you are still unable to send or receive SMS, follow these steps

1. Make sure that the contact who's trying to message you isn't blocked in Settings > Messages > Blocked.
2. Make sure that you are using the area code with the contact's phone number. When sending messages internationally, you also need to use the contact's international code.
3. If using a "Pay as you go" SMS plan, it needs to have enough available balance. Contact your carrier if you're unsure of your SMS plan or the available balance.
4. If the issue occurs with a specific contact or contacts, back up or forward important messages and delete your current messaging threads with the contact. Create a new message to the contact and try again.
5. If the issue occurs with a specific contact or contacts, delete and re-create the contact from the Contacts application. Create a new message to the newly created contact and try again.
6. Back up and restore your iPhone as new.
7. If your carrier has recently ported your phone number, the porting process may not be complete. Contact your carrier to confirm that the porting process is complete.
8. Contact your carrier to confirm that you are provisioned for sending SMS and are in an area with cellular coverage.
9. Contact your carrier to verify there are no blocks or filters placed on your wireless account preventing you from sending SMS.

Thanks for reaching out to Apple Support Communities.

Cheers,

Pedro.