Add Call / Conference Call Not working for some phone numbers

I am having a problem "merging calls" (creating a conference call). The problem only occurs with some phone numbers which do not count the call time (i.e. I have it working on my phone with regular phones).

For example, I was trying to help a family member get a credit report through TransUnion's customer service number of (866)-744-8221, but when I go to "merge calls" the button is greyed out. It looks like this occurs because the iphone does not think the second call has picked up and instead shows "(866) 744-8221 calling ..." the entire time I am on a call with the number (instead of the usual time count up).

Is there a fix to make the iphone realize the other end has actually answered?

Thanks!