Are there any open source dialer apps for Android that have call recording function?

This may or may not be helpful, as I couldn't find any sources to download an APK, but GrapheneOS' default phone app does call recording.

LineageOs has it too
I run Graphene OS on my phone and can confirm that the OS dialer app does have call recording.
- What? Tf Im also on graphene.How do I use it?
I believe the built in MIUI dialer still does.
not sure how it would work I'm on graphene myself, maybe you can install their apps apk and select the dialer from there.
- The dialer doesn't seem to be included in their apps repository, sadly.

I know OP is looking for open source but for anyone else, ACR Phone works wonderfully if your phone is rooted. It requires an additional Magisk module but once installed the app is amazing and highly customizable.

I don’t know of any, I know that at one point you could enable it on one plus devices since outside the US it has that functionality built in. There was an app called j.one.plus.tools that did it IIRC.

The upstream dialer might have some functionality like that built into it too.

LineageOs has it too

There are few good reasons to record phone conversations

There are lots of good reasons to record phone conversations, such as holding others legally accountable for their legally binding verbal agreements. Most places have one-party consent laws, where if a single party in the conversation consents to being recorded, it's legal.
- I say the same thing they say to me. I'll be recording this call for quality assurance.
- such as holding others legally accountable for their legally binding verbal agreements
  
  Nonsense. Do you record a lot of legally binding verbal agreements on your Android phone? I'm certain that you do not.
  
  Anything worth agreeing in a "legally binding verbal agreement" is worth making a written agreement. It's been a long time since I studied law but IIRC verbal agreements are only really enforceable where they're commonplace agreements and they are at least partially performed. For example, "if you pay me $x per month you can put your cows in my top paddock" is legally enforceable, but having a recording of such a conversation won't make it so - it's already enforceable once the cows are in the paddock.
  
  If you want a more complex, contingent agreement like "you need to pay me $x if it rains tomorrow", it's not really enforceable in any practical sense. A recorded verbal agreement would need to be very long winded to cover all the edge cases... how much rain, for how long, where does it need to rain, what times are "tomorrow", does "rain" include sleet, what about hail, et cetera. Of course - this type of agreement is always written down.
  
  Additionally, I acknowledge that this hasn't made it into case law yet, but since the advent of deep-fakes is this type of recording going to be reliable in court? "I never said that" seems like a plausible defense - so again, it's going to come back to performance of the contract.
  
  Finally, agreements worth making tend to involve a lot of nuance. There's a courtship, a dance, a tête-à-tête, asking someone whether you can record them is just plain weird. The act of trying to record someone is likely to influence the agreement itself.
Lol Comcast. You'll want to record every. fucking. Phone call you have with them.
- I wonder whether having a recording has ever helped anyone when dealing with comcast. I'm doubtful. They're a large sophisticated organisation with an army of lawyers who will just do whatever they will do without regard for your recording.
Some people just have shitty memory so it's nice to be able to go back and listen to stuff that was missed.
- Do you actually do that? Have you tried it?
  
  I'm a consultant and discuss complicated things with clients several times a day. Exactly the type of conversation that will include bits and pieces that people want to take note of.
  
  If someone asked my permission to record a conversation I would decline and tell them that I'm probably not the best fit for them moving forward. I'm certain that most other consultants in any professional field would do the same. The reason being simply that it's a huge red flag - such a client is guaranteed to be a huge pain in the butt - much better to spend your time with other more co-operative clients.