Practically every cellphone I've ever seen, period, has been able to change it's signals one way or another, and most cells after at least, say '96, has been able to get new ones to that, so I would say it doesn't really matter so much as long as you stay true to what kind of sound-emitter it has.
Take for example an old Nokia 6110 from '97. It has a very simple piezoelectric element and a sound circuit unable to do anything but simple beeps. Still, the traditional Nokia-tune aside they didn't even compose themselves, it's practically impossible to tell it apart from for example contemporary Ericsons, Philips, Motorolas, DanCalls or any other cell from that generation. You might, if your really anal about cellphones on a near autistic level (not counting us actually working with sound, but we have to be) hear different brands and models in real life, but as different miking techniques and microphones make the same unit sound their very own way, it's practically impossible to tell one phone from the other on-screen.
The next generation cellphone signals, the polyphonic ones, had yet another quite unique sound as most of them, as far as I know, are based in FM-synthesis, and has pretty primitive elements. The sound was a very lofi and round sound, often mimicking marimbas, bells, glockenspiels, xylophones and other equally annoying things when left all alone in bad quality.. Like the old Commodore 64 they could be made to performe samples, but they wasn't made for it be far. Here's the first time the phones actually began changing appearance more clearly, like being able to fold in the middle and slide up the entire front panel. Still, noone seems to actually remember that era as it wasn't that long at all, and the step over to the third generation of ringtones wasn't, in the eyes of the common man, that big.
Now (early 2000's), the signals became harder to snatch directly from the phone. Before, the beeps was totally generic and had absolutely no artistic value; you could record it any way you wanted, but you couldn't use the tunes, so you had to do something own of it to avoid a potential shit-storm.
With polyphonic signals you can't just record anything freely anymore as it's not generic but heavily designed. You can, however, probably use songs you program yourselves in these cellphones, but personally I don't think it's worth the risk as it's even easier just playing the little tune on a synth. I normally use my rebuilt Commodore 64 or Ensoniq SQ-80 for this!
The latest generation, the PCM-based, makes a return to designer heaven again as it can take absolutely any sound you make and just output it, colored and all, through the surprisingly well performing small piezo elements for you to record any way you see fit.
All these three different generations of cellphones has been totally different from each other, but one thing has always been consistent; they all has had a simple bleep. ALL of them.
With that said, you shouldn't need to worry too much about credibility. As long as you use bleeps it will always work no matter which cellphone you use, but it can ofcourse be a little dull not giving different characters very different signals.
In that respect, you still have much artistic freedom as long as there's no extreme close-ups on the phones or such. A single row monochromatic display gives away it might have been found in the hands of a dinosaur skeleton somewhere, as does an antenna. But for antenna-less cells without a visible screen that actually fits in a pocket you can do pretty much whatever you want.
Regular phones, on the other hand, are more critical. One look and you know instantly what to expect. You needn't worry about keeping true to the brand, but you MUST stay true to the technology. A round dial phone will always have bells, mostly of brass or steel, a phone with the same shape as a dial-phone but with buttons will have a very muffled sound, often quite shrieky, and combined phones with faxes will sound like an old modem.
In short; it doesn't matter that much how close to reality you are, this is film, here we strive for credibility, not much else. You must be respectful to the sounds, for example a Nokia-Tune on an old Bakelite dial-phone sound eighter very funny or plain retarded depending on the occasion, no matter what it will pull all attention to it immediately, but a non-generic sound played through an iPhone and used on an Android LG might very well sound more convincing that would the real one. I do that all the time, though I have a collection of about 15 cellphones in my studio ranging from approx 1990 (a Motorola MSN) to 2010 (my present black iPhone 4_3G) :-)
And yes, I always do all my ringtones myself. I'm not willing to risk getting in trouble over a frigging cell-tone... But I often use my real cellphones, com-radios, transmitters and broken TV-sets as both sources and heavy processing tools for my sounds, even faked radio communications, public address and SciFi information-systems to name but a few :-)