How do I fix merge function for conference call on my iPhone?

Contact your carrier. Multiway calling and its limitations are determined by settings in your carrier account.

As stated, multi-party calling is a carrier feature.

If it's supported on the iPhone by your carrier and it doesn't work, they may need to re-provision your line.

Call them back and escalate the call until you get hold of someone who knows what that means.

reg204 wrote:

I did and they don't have any limitations/restrictions. The merge functionality is not lit and I don't know why? I searched for a selection in settings??? No joy!?

Call them back, ask for a senior tech, and have them reprovision your number.