1. Forgetfulness. This applies when the perpetrator is busy with something else, e.g. sleeping, and in turn forgets to return the call or reply the SMS.
2. Laziness. Hardly the most forgivable, as it almost deems the victim's SMS/call as unimportant. Does paint a hazy picture of the friendship.
3. Stinginess. "Because my mobile plan has only 360 free SMSes."
4. Excuses. "Because I was (insert random activity here)."
5. Avoidance. Simply put, to avoid talking/contacting a specific someone.
[More to follow upon conceptualization and further research]