I do not do phone calls. Many experts believe it is difficult to keep litter mates but the actuality is that raising any two puppies together that are similar in age can lead to some issues with training. I've done it several times since I tend to keep females for my kennel.
It does make it harder to house train because ideally, a pup should be taken out as soon as they wake up. If you have two or three, you can not get all of them out at the same time. If you do manage to get them all out at the same time, half the time they are running around playing instead of eliminating. In addition, part of house training is rewarding them when they eliminate outside, but if you have more than one, that can be difficult.
Of course, when you start obedience training, you have to train them separately first, then train them all over again in groups of two and finally all together. In addition, if you have an "accident in the house" you have no idea which one to blame. On a good note, the pups usually determine who is the dominant pup as they are maturing and you really don't have some of the same assimilation issues as when you bring a dog into a household.
If you want to keep more than one, definitely do it, but be aware that training is going to be more difficult especially house training.