You have an XY Problem. You should be using the official communication channels to communicate with the other company, not using your "personal contacts" instead. If you do not know the official communication channels, ask your manager.
If he doesn't know either, tell him that your work cannot progress until you contact the other company, and let him figure out how to fix it. It is a part of a manager's job to facilitate communication and remove obstacles impeding the team's work. Of course, you could offer to assist him by sharing the information you have gained thus far.
It is never a good idea to do anything which catches your manager by surprise. Keeping your manager out of the loop when you contact people outside your company is most certainly not a good idea. This does not mean you should put your manager in cc on every mail you send to the other company, but just that you should keep your manager updated about the communication.
Many companies prohibit their employees from contacting employees of client/vendor companies directly, for a number of reasons:
Avoid bypassing management's priorities: When requirements come through the official communication channels, it is easier for management to prioritize the tasks. Every customer wants their issue to be given the highest priority, but nothing will get done when everything is urgent. Management assigns priority to prevent such chaos. By contacting the lower level employees directly, you bypass the management, which can create problems.
Avoid perception of preferential treatment: Allowing personal contacts for communication means companies that do not have such personal contacts in the company will have to wait longer or get inferior service. Even if that is not true, the company's other customers will carry that perception if they find out about this personal contact arrangement.
Official contacts are well trained for the job: Why do companies send spokespersons to press conferences instead of any random employee? How do these spokespersons respond "no comment" without batting an eyelid to controversial questions? Representing your company to outsiders is serious business, for which the company trains them thoroughly.
When things go well, using personal contacts instead of the official channels doesn't do any harm, but when things are not so rosy, such personal contacts can create problems for the company. What happens when the personal contact doesn't give you the required information, or gives the wrong information? What if you say something rude which damages your company's relationship with the other company?
Prevent accidental divulgence of confidential information: Personal contacts in companies have varying levels of awareness of corporate policies. A somewhat naive employee could unintentionally divulge company confidential information to outsiders.
"But hey, I am just asking my personal contact for contact information! It is no big deal, right?"
The reasons stated above may sound paranoid. None of these may be an actual problem with your approach, but you still have a couple of problems:
Violation of Contract: Your personal contact and you could be violating the contract between your companies. To avoid controversies, such as the above points and worse, companies prefer to specify the communication protocol in the contract, as service-level agreements (SLA).
Unreliable communication: If the only interface between your company and the other is you and your personal contact, your project is taking a huge risk. If either of you leaves the job or goes on vacation, then the project will slow down to a crawl or even grind to a halt.
Bypassing metrics: Management loves to track "metrics", especially in deals with other companies. By using your personal contact to get information from the other company, you prevent the management from getting the metrics they might be interested in. At the end of the project, they might be "pleasantly surprised" to find out that they have raised a grand total of zero issues with the other company, which is misleading.