I have a iphone 5s and my calls fail

Howdy saravargas1.

If I understand correctly you are able to make phone calls but they seem to disconnect when you are speaking to the other party. I would use these troubleshooting steps from the following article to help troubleshoot that issue:

If you can't make or receive calls on your iPhone

Update your software

1. Check for a carrier settings update.
2. Check for an iOS software update. Some updates might require a Wi-Fi connection.

Remove and reinsert your SIM card

If your iPhone has a SIM card, remove the SIM card, then reinsert it.

Update your Preferred Roaming List (PRL)

The PRL determines the cellular towers that your iPhone uses for cell service and selects those with the best signal strength.

• If you're on the Verizon network, dial *228 and select option 2.
• If you're on the Sprint network, dial ##873283#.

Contact your carrier

When you contact your carrier, check the following:

• Your account is set up to use your iPhone.
• There aren't any localized service outages.
• Your account doesn't have a billing-related block.
• Your calls don't have errors on the carrier system.

Check your network settings

1. Reset your network settings. Tap Settings > General > Reset > Reset Network Settings. This will erase all current saved settings, including Wi-Fi passwords, preferred networks, and VPN settings.
2. Try to make or receive calls in another location.
3. Switch to a different network band. Tap Settings > Cellular and turn off Enable LTE, 4G, or 3G (this option will depend on your carrier and device model).

Thank you for using Apple Support Communities.

All the best,

Sterling