Hello there, Vineethk.

It seems you have been unable to make conference calls since upgrading to the latest version of iOS 9. The following link to the online iPhone User Guide provides the steps to review for making conference calls:

Conference calls - iPhone User Guide

Conference calls

With GSM, you can set up a conference call with up to five people (depending on your carrier).

Note: Conference calls may not be available if your call is using VoLTE (Voice over LTE).

Create a conference call. While on a call, tap Add Call, make another call, then tap Merge Calls. Repeat to add more people to the conference.

• Drop one person: Tap next to a person, then tap End.
• Talk privately with one person: Tap , then tap Private next to the person. Tap Merge Calls to resume the conference.
• Add an incoming caller: Tap Hold Call + Answer, then tap Merge Call

As far as troubleshooting goes, this Knowledge Base article offers up great troubleshooting tips for issues with making calls:

If you can‘t make or receive calls on your iPhone

Check your iPhone settings

1. Turn airplane mode on and off. Tap Settings > Enable Airplane Mode, wait five seconds, then turn airplane mode off.
2. Check your Do Not Disturb settings. Tap Settings > Do Not Disturb.
3. Check for any blocked phone numbers. Tap Settings > Phone > Blocked.
4. See if Call Forwarding is turned on. Tap Settings > Phone > Call Forwarding.

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.

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).

Still can't make or receive calls?

If you still can't make or receive calls, restore your iPhone as new. If restoring doesn't fix the issue, contact Apple Support or take your iPhone to an Apple Retail Store or Authorized Service Provider.

Thanks for reaching out to Apple Support Communities.

Kind Regards.