A VPN (Virtual Private Network) builds an encrypted virtual tunnel across the internet to improve the security and privacy of your traffic. The encrypted stretch runs from your own device to the VPN server; beyond that point the traffic travels over the ordinary internet. In other words, a VPN protects the leg between you and the VPN server rather than the whole path. Keeping that distinction in mind is what stops you from misjudging what a VPN can and cannot prevent.
There are two main phone-related uses. The first is protecting traffic on public Wi-Fi. On the open networks found in cafes and airports, other people on the same network may still be able to observe your traffic. That said, websites and apps now encrypt their own connections as a matter of course, and the audio of most VoIP apps is encrypted too. What a VPN adds is the ability to hide who you are connecting to from the operator of that local network and from anyone sharing it. The honest framing is not "with a VPN on, anything I do is safe" but "one more layer of information is now hidden".
The second use is corporate remote work. Connecting into the company network over a VPN lets you reach the in-house phone system (PBX) from home and use extension calls and internal systems securely. Cloud-hosted PBX systems can also be reached over the public internet, and some organisations run remote extensions without a VPN at all, while those with stricter security policies still require one. Bear in mind that routing calls through a VPN sends them the long way round, which adds latency and can leave a noticeable gap before you hear the other party. If extension calls alone sound wrong, the VPN path itself is worth suspecting.
VPN services come in free and paid forms. Free ones commonly cap data or speed, and some operators have been reported to collect subscriber activity and use it for advertising or pass it to third parties. Paid services typically advertise a "no-log policy", meaning they claim not to retain records of your traffic, yet there is no way for a subscriber to verify that claim directly. A practical way to compare them is on the points you can check from the outside: whether the policy has been examined in an independent audit, whether the operating company states its location and contact details, and whether the cancellation and refund terms are clear. See phone number privacy for related guidance.