The ability to place a phone call on hold is not a new feature; it has been available in mobile telephony for decades.
The important point, however, is whether your cellular service provider and network support the feature.
On an iPhone, placing a call on hold, switching between two calls, merging calls into a conference call, and managing multiple calls are standard telephony functions provided through the iOS interface. However, iOS cannot independently enable these capabilities if the carrier or network does not support them.
In other words, iOS provides the interface, but the cellular carrier provides the underlying network functionality.
Therefore, if you experience problems with call hold, call merging, or conference calling, it is not necessarily an iPhone or iOS limitation. The first thing to establish is whether your cellular carrier supports the particular feature and whether it is enabled on your connection.
It is important to distinguish between a feature being available in iOS and that feature actually being supported by your network.