All CRMs have their limitations. I think one way to mitigate these limitations is to develop your own modules or apps that interact with the CRM that cover the gaps.

The interaction can happen using REST APIs either on the CRM side or your own app.

For example, if a user enters a new contact in the CRM, I want something to happen that is specific to my company. For this to happen, the CRM calls an endpoint in my app sending the contact's info in the request.
The CRM therefore needs to have the ability to define a url that is used to make the request to it every time a contact is saved.

Also, this should work in the opposite direction. I want to call an endpoint in the CRM to save a new contact. For example, I entered a contact in Excel and Excel can send this contact information to the CRM to be entered automatically by calling an endpoint in the CRM.

I was wondering if there's an open source CRM that supports these scenarios. A CRM that has a lot of API endpoints.

The goal is not to modify the CRM'ss code to add new features but to have it interact with my own app which has the extra features. This way I am in total control of the new features.

If you're unhappy with your CRM and have special requirements, why don't you switch to an open source one and have AI customize it for you? I see people say their CRM doesn't do this or that and they're still spending too much time doing manual work in it.
In this case, customize an open source CRM to address these issues.

Fork the code and maintain it your way.

I know one of the pushbacks is that they don't want to host it themselves or maintain it.
In this case, why don't you hire someone to customize it for you? Let them use AI for that. It will make it a lot cheaper for you and you get the new CRM a lot faster. You test it heavily and if it works to your satisfaction, you tell them to host it somewhere, and you pay them. This is a one time thing.
Then you pay a small hosting monthly or yearly fee to the host. There won't be any per user fees or storage or per X fees. You get all the features for a simple hosting fee and with no limitations.
The hosting company will backup the database for you. Your technical concerns can all be addressed by the hosting type and features. Redundancy, scaling, backups... whatever.

People keep saying you have to maintain it. Once it's running, you don't have to maintain anything.
If an unexpected problem rises, tell the person who did the work to look at it.
If you have a technical person at your company, they can probably fix it.
The person who handed off the new CRM can train you or your technical person.

Every company has its own needs from a CRM, some of which are very specific, and therefore there's no CRM that can do everything you want. Then you will have to create your own CRM or customize an open-source one.

Twenty is an open source CRM with about 55k stars on Github.
Their website mentions the features and how good it is.

I am also interested to know what it can't do and its shortcomings.
If you tried it and didn't like it, why and what turned you off from using it.

I have seen people use apps like Word, OneNote, Excel and other note taking apps for taking notes outside of the CRM. What's your use case and why aren't you doing it inside the CRM? Is the CRM software lacking a notes section?

If you're making a call to a regular phone number in the US, how is the call getting recorded, transcribed and summarized back into the CRM? Is a VOIP system or a dialer being used?
Is Whatsapp being used?

I am asking about the recording part in particular when making a regular phone call. I know once a recording has been captured, voice to text and AI can do the transcribing and summarization. Does the CRM have like an api to receive the text including the metadata, so it knows where to place it?

Most of the CRM software is web-based. There are some free open source ones like EspoCRM. Some companies shy away from them because they require IT skills, monitoring, maintenance, backups, which are hassle or expensive for small companies.

If you're a solo CRM user or a small company with a few users, would you use a free desktop CRM software if it were as easy to install as any desktop app? If you're the only user, the app and the database will be on your computer. If you were a company, each user will have it installed on their work computer. The database can be hosted anywhere. A computer in the company with no hosting fees, or hosted in the cloud (Azure or AWS) or hosted at a hosting company with as little as $5 for shared hosting with backups. This option will be Windows only, so no Mac or Linux.

There will also be a web based version with the same features.

But I am inquiring right now about if there's interest in the desktop version if it means it's hassle-free to install, host and use. It's free as well.

The business model is making money from advanced customizations.