Somedays, I can't read the writing on the wall

Detail:
This picture was taken near Fudan University in Shanghai China but tags similar to these seemed to be all over the city, and, to a lesser extent, all over China. They are tagged on walls, street curbs, they are on stickers on telephone poles, these messages cover sidewalks.

I asked a Chinese person I met at an English chat/practice what they were for and apparently, they are all phone numbers plus, "I will dig a hole for money." He says generally speaking they are all about "dig a hole" though some are more along the lines of "have tools will work."

But be careful, my friend cautioned me, these are all scams. We say, "these are people you can't believe in."

Unfortunately, his English wasn't good enough for me to get anymore real information.

What kind of scam starts, "I will dig a hole for money." ?

Any ideas?