There are a few different services out there you can use for entertainment. My personal favorite is calling 719-266-2837 and I can't explain why but I love it deeply. Another classic is "Always free, often busy" Dial-A-Song from They Might Be Giants. Lately this fun phone number phenomenon has given rise to numbers you can text too. Winning in the "Text this number for funsies" category is a number mentioned by Serenity Caldwell over at iMore in an article about having a little fun with iMessage. I won't give it away, I'll just tell you to text any sort of message at all to 866-740-4531 and delight in the reply.
Check It Out: Amusing Phone Numbers To Call And Text