Some carriers, especially in roaming and in regions with VoIP carve-outs, block voice and video over messengers while leaving text and media alone. The text "are you there?" arrives. The call button does nothing. Switching the entire phone to VPN works, but eats battery on a long international trip and slows everything else.
With QPOL split tunneling you label just the messenger apps. Voice and video go through the tunnel; everything else — maps, taxi, banking, work email — uses the carrier's normal path. Battery impact is bounded to the apps you tunnel, not all background traffic.
Pick a node geographically close to your contact for lowest latency. A Berlin node for a Europe-Europe call is fine; the same node for a call to São Paulo will sound underwater. The directory shows latency per node so you can pick by ear.
For "the call drops every two minutes" reliability problems, try a different node before assuming the protocol is at fault. Some intermediate carriers are unstable for certain UDP patterns; switching exit usually fixes it. The starter balance is enough to test several nodes on the same network without paying for a voucher.