If you are concerned that people may be able to hear your conversations or hack into your call remotely by listening in to them it is certainly possible that with the right equipment, a lot of knowledge and a good incentive, that someone may be able to do so; just as it is with both mobile and ordinary landline telephony.
However, there are a few things to consider. Firstly, you need to ask yourself why anyone would be remotely interested in your conversations? Secondly, if you didn't worry about it before, when it was possible for someone to simply put two clips across your telephone wire to listen in, why are you concerned now when it requires a lot more technical ability?
You also need to separate in your mind the difference between a phone call and other pieces of computer information sent down your telephone line. A phone call happens in real time, its start point is unknown before it happens and is gone forever when it's over.
Other data, such as emails, are stored in ordered format and can be searched for historically and worked on over time. So phone calls start and finish more securely than most other communication methodologies and don't leave a stored record of their content.
If you're making telephone calls that need to be totally secure from eavesdropping you should use no publicly available telephone service.
However, unlike ordinary telephony, VoIP can be encrypted to make it secure but unfortunately there are some very misleading statements being made about secure or encrypted VoIP. To be any use at all, encryption needs to be end-to-end in order to fully protect the conversation.
But currently the only way that this is possible is on a VoIP to VoIP call over the same vendor network using hardware, which supports it and a network, which allows it. This is a rare kind of phone call. The overwhelming majority of calls going to and from companies originate or terminate on the public telephone network. The PSTN is not encrypted, so any calls placed to or from it are unencrypted - there is absolutely nothing any telephone service provider can do about this.
Furthermore, calls from one VoIP network to another VoIP network are also not encrypted which leaves the only use for encryption to be for in-company calling. Sadly, an attacker with access to the phone's local network will be capable of disabling encryption but if the local network is secured, there's little to no benefit to encryption.