My father has an old landline number that used to be associated with a business he shutdown more than a decade ago, but he puts up a fair bit of resistance when my mother encourages him to cancel it because he insists there are dozens of people who still might try to call him at that number.

Okay, fine. Lets assume that is true.

He recently switched from an old Lumia to a Moto G which has a built-in SIP client. If I port that number to a SIP provider would the incoming calls come through seamlessly? I also note that VoIP.ms discourages using it however it refers to an ancient version of Android.

My attempts to get him to use Google Hangouts for something else were unsuccessful because it took him too long to unlock the screen so I am not inclined to attempting this with a third-party softphone.

Is anyone using this successfully or is it likely just to generate frustration?

Reading over the reasons why Android isn't "100% supported", this looks likely to end in frustration. (Though I have not tried on an Android, nor do I know if the capabilities of the SIP client have improved.) I know you said you weren't open to 3rd party apps, but I highly recommend Bria for SIP on a mobile phone. Better control over codecs, and more important, the ability to tune settings to ensure the app stays registered. If the native Android client has issues staying registered, this would result in missed inbound calls.

Or perhaps an even easier option: port the number to Google Voice (or your VoIP provider of choice, like voip.ms) and have it forward to his cell. No need to bother with SIP if this is all as infrequent as it sounds.

Thanks for the suggestions,

I was leaning towards SIP rather than forwarding so he could theoretically at least make an outgoing call.

I actually use Google Voice myself because a former employer offered me the opportunity to port out a cell phone number I had used since 2004.

Do you want to get rid of the lindline to save money? Or some other reason?

I got rid of mine because of the expense but I wanted to keep the number, so I ported my old landline to Magic Jack a few years ago. The service has been fine and it's cheap. I just looked and it's at $35 for the device and a year of service, plus $99 for an additional five years. I'm sure there was a fee to port the number but I can't remember how much it was at this point.

I've got the Magic Jack connected to the house's wiring and it works well; I can plug a phone into any of the existing jacks just like I could back when I had POTS.

For my mom's purposes it is about cost and that virtually all the calls to that number are from toner scammers. For my purposes it is more about making what my parents use mobile friendly because they spend more and more time away from the house each year.

I'd echo the Google Voice idea, even though I've been running SIP for 15+ years myself. Google Voice also would let you better filter the calls, especially with many of them being scams. You can make outgoing calls with it as well, which takes care of that issue.

On a mobile phone, SIP uses quite a bit of horsepower, ergo, battery, and SIP isn't real tolerant of spotty connections.

Sadly native android SIP functionality was never really a priority and has fallen by the wayside. Bria (or Softphone on iphone) are best mobile solution