Zoho Mobile: How to link and activate Mobile calls automatically to sales opportunity?

Zoho Mobile: How to link and activate Mobile calls automatically to sales opportunity?

OPPORTUNITY
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I have a business opportunity XX

I can not make phone calls from my mobile.

The phone icon at the bottom is not active.

CONTACT
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I make a phone call with my mobile using Zoho CRM app

And the mobile app asks me if I want to register the phone call.

It works using the contact but not he opportunity.

I want to link the phone calls to the opportunity not the contact and I cant because the phone icon is not active

Is it possible? How?

Thanks in advanced for your feedback and help

Regards

Salvador