Actually, iPhone won't allow exporting contact to SIM card. So you have to reach this aim with the help of Android phone. Just follow below steps.
This can be a good way if you only need to transfer a few number of iPhone contacts to SIM card. You can do this by sending emails of the contacts. You can email the contacts one by one to your Android smart phone, and then import them to the SIM card in the Android phones. The detailed steps may better help you.
Step 1. Go to the Contacts app on your iPhone, find those contacts you would like to transfer to SIM card. Then tap "Share Contact" and share these contacts via email.
Step 2. Download the vCards you just shared via email on your Android phone. Go to Contacts app on Android phone, then you can choose "Import from USB storage".
Step 3. Go to Contacts app to check if contacts have already been imported to your Android phone. Then go back to "Import/Export contacts" again, this time you should tap "Export to SIM card". Of course you need to put the SIM card into the Android phone before this step.
With this way, you can copy a limited number of contacts from iPhone to SIM card. This isn't a direct and simple way to transfer iPhone contacts to SIM card. You need the extra help of an Android phone, and you also need to type emails one by one to import SIM contacts. Although, it is a good way to back up the contacts by coping them to SIM card, you can also use other backup methods to save the contacts, like use data backup programs or back up them to the cloud.
For more details, check: How to Transfer Contacts from iPhone to SIM