CallTap records phone calls with the help of the 3-way conference feature provided by your mobile carrier. After you add CallTap as a silent participant to a call, everything everybody in the call says will be recorded until you hang up.
3-way conference calls require support from your mobile provider. If you notice that CallTap is recording silence, that's a sure sign that the calls did not merge successfully. This is most likely because your mobile provider does not support 3-way conference calls. Please give them a call and request they enable 3-way conference calls for your line.
If your mobile service supports 3-way calling, another reasons why sometimes merging calls fails and CallTap records silence is issues with WiFi calling. Please turn off WiFi calling and try again.