I've had a few friends and family members (cousins) marry by now. It's not unusual for Dutch brides and grooms to make a wishlist or express wishes to wedding guests. As far as I'm aware, the proper etiquette is that you stick to their wishes or you give them money so they can freely spend it on items from their wishlist after. In your case, this would mean that gifting them the monetary value is okay, as long as you don't mention it is to be spend on a phone, and on a new phone only.
In case of gifting money/monetary valure, freely spendable money is preferred over gift cards, though gift cards that can be spent in a lot of different places are more acceptable than ones that can only be exchanged in one (kind of) shop.
Another thing is that when giving money or gift cards, over here it's really appreciated if you do something funny/creative with the gift wrapping. Putting some care in the wrapping/presentation of your gift shows you care about making their day festive, and you aren't just "paying an entrance fee" for the party. Usually people get quite creative with the 'gift wrapping': I've seen 'money trees', stacks of >100 envelopes with only a few actually containing money, coins frozen in ice, origami folded bills... I've put together jars like this one that are pretty popular.
So, in your case, you've already made up your mind about how much you're going to give the newly wed couple: the monetary value of a new phone. The least rude way to do so is to just gift that monetary value in the way that allows the newlywed couple the most leeway in how to spend it (in your case, the check is more preferable than the gift cards, which are more preferable than just giving them a single phone).
Don't put any stipulations on the gift: Don't mention what you want to see them use the money for, let them choose for themselves what they spend the money on. They might have other priorities that you're currently unaware of. And for that extra bit of love an care, don't just hand them a check or stack of giftcards, but wrap it up in a fun way.