Is it possible our VoIP calls being redirected from our intended phone number to another phone number unknown to us? Our VoIP number is currently forwarded to mobile phones and computer phones. Would this be an issue with our VoIP provider? Our local network or our mobile phones?
In a situation like this, you don't know what attack vector is being used, but it seems that you may have some anecdotal evidence from customers?
The best solution is to log all occurrences and then attempt to determine the threat(attack) vector. You may have to reach out to the companies that provide your phone service or the client's. This is very dependent on how the attack takes place, considering multiple client's are potentially vulnerable with an unknown breach. Once you have a log of what user's are doing, and what results they are getting, you can quickly identify what companies are doing it and can, with the help of ISP's or phone companies, determine where the attacks are coming from.
You can start by testing using google voice numbers or track phones you activate and call or attempt to contact yourself with.
I would also invest in a VPN service and turn off location services while you do this so that an attacker might not know that you are the one making the calls if they have you blacklisted for the malware.
Once you have even a small record of where the attack is coming from, you can begin to figure out how it is happening. Vague suspicion isn't enough to figure out what is actually happening.
The more specific the threat, the more specific the solution will be.
It's not a common practice, but I happened to solve three cases of the exactly that problem. Without a detailed information and a closer look I can't tell you what has happened exactly in your case, but here are the most common pinpoints: the phone firmware, the software phone exploit, mitm for the VoIP client and the PBX exploit. If you want further help - I will need to take a way deeper look at the case