For at least two years, I have received calls from 970-931-4758 and other phony numbers offering to lower the interest rate on my credit cards.   Attempts of block have proved useless.

Just got a call from this number. I answered, heard nothing on the other end, and after 15 seconds they disconnected.

Called them back right after to find out the number wasn't in service or whatever.

I know some calls like this are a form of war dialing--not sure if this is too.

Received a call from them minutes ago, but did not recognize the number so did not answer my phone. They did not leave a message. Will block this number now.

What you experienced is exactly they way a predictive autodialer works when more calls are answered than there are agents available.  You can file a complaint with the FCC for a dead air call, ie no agent ever picked up before the call was disconnected. By law, autodialed calls my have no more that a 3% dead air rate. BTW, autodialed calls to cellphones are illegal except in special cases.  The giveaway that it is an autodialed call even if an agent comes on line is the delay before anyone is there.

Called my cell. Didn't answer. No message.