I will leave aside the cheating thing, because I think it has been discussed plenty in other places (but for the record, I am with RedPepper on this one, entirely!)
I have issues with cell phone etiquette in general. Sitting having a conversation with someone and the phone rings and their instant comment is "I've got to take this" when they don't even know who it is (believe me, I checked - they didn't have custom ringtones or anything like that). WHY do you have to take it? Is talking with some random person who is calling you more important than talking to me, since we are out together? Says a lot for how you value our relationship, doesn't it?
As far as texting with a lover is concerned, I think it is something that the individual relationships need to work out. For some it's not such a big deal, for others it is. I can not turn off my cell for long periods due to work commitments, but for whichever of my partners I am not with, they understand that I am not instantly available. I will more than often ignore text messages until a convenient time comes up - I won't interrupt a conversation to look at it and reply.
One of the things we have set up is the idea of quality time, whether it is a date night or whatever. During that time, there is no communication with the other, unless there is some dire emergency (and I do mean DIRE). But in lesser ways that applies to watching TV with one of my partners, too, even though that's not official "quality time".
Bottom line - if you are upset by what is going on then you have every right to express that and work with them to come up with an etiquette that works for all parties.
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