What is your use case, exactly? Are you trying to do some complex programmable telephony, or just make/receive calls across your personal devices?

If the latter, I think just having an iPhone and iPad or Mac will let you natively do that: https://support.apple.com/en-lamr/102405 without needing to route the call through some rando internet service.

Otherwise, there's still Google Voice, but the call quality (last I tried) was consistently mediocre to terrible.

Vonage is still around too, providing similar VOIP services to Skype: https://www.vonage.com/unified-communications/pricing/

If you want a brand-name SIP provider, Twilio offers those services: https://www.twilio.com/en-us/client/pricing

SignalWire too: https://signalwire.com/use-cases/voip

I think at the end of the day, VOIP call quality is typically too dependent on factors outside of your control (codec, bandwidth, network congestion, etc.). And there's going to be additional latency since they have to route voice calls out and through some data center and to you. If there were an easy way to up the quality consistently, probably the business providers would've done that long ago and upcharged it as "Premium Voice" or such. The cell carriers did just that with "HD Voice" and such, but that's only when they can control both ends of the conversation. That's also why purely-internet voice comms (Discord, Zoom, etc.) can sound so much better; they have end-to-end control of the traffic. Generic VOIP/SIP providers that terminate to POTS don't have that luxury.

This is a great Ask HN, I'm excited to read any input the community has to provide. To be totally honest though, I didn't realize Skype was still a separate service.

I'm not personally aware of any SIP / VOIP solution that would include specific Apps and such - love to know more!

IMHO, most people using SIP providers are going to hook it up to something that feels like a traditional phone. Maybe that's a ATA that provides a FXS port to plug in a regular or cordless POTS phone, or as the backend for deskphones for a business. I've been using voip.ms for a long time for this; despite the domain, I think they're based in Canada. I think they got some investment recently and they've rebuilt their website to be more flash and less substance, but service continues as-is, at least for my minimal usage (except for the time they and a lot of other voip providers were getting DDoSed; that was kind of rough).

They've got a wiki about using their service from a smartphone [1], but I'm guessing it's fairly dated; I don't think Android has a sip client integrated into the dialer anymore? And with Doze and modern networking, I don't think you're going to have a good time with a plain SIP client, if you want to get unexpected inbound calls; and for iOS, you always needed a server assist to get a push notice for unexpected inbound calls. I don't know if a regular SIP service is going to let you roam from wifi to cell and vice versa during a call either. If you want things to work well, you really need an intermediate server built around how modern mobile apps can communicate ... and then you get awful close to just using one of the OTT messengers and whatever calling in that app.

Voip.ms also has a more recently published guide [2] in their blog... so I dunno, maybe? Vonage claims to have an app for android/ios for their business communications customers [3] that might work, too?

[1] https://wiki.voip.ms/article/Android

[2] https://blog.voip.ms/blog/voip-on-cell-phones-ios-and-androi...

[3] https://www.vonage.com/downloads/

Just let go on these spyware.

Use Jami! https://jami.net/

Why not use Signal? You can message and phone call securely, it's free and not too feature packed.