Here is a tactic. Record a new voice mail using SIT (special information tones) from this web page: https://en.wikipedia.org/wiki/Special_information_tones
You want to use the Intercept or IC tones
While you are recording your voice mail message, start recording, play the tones twice and then give your normal greeting. Text your contacts and let them know that you are having some problems with the phone company but if they get your voice mailbox, go ahead and leave a message anyway. Answer only those calls that you know and let all others go to voice mail.
The tones tell the auto dialer at the telemarketer, that your line is no longer in service. Normally, after 1 to 3 attempts at calling your number, they will remove your number from their database. If you are being hounded by several telemarketing firms, it can take a couple of months but you should see a rapid drop off after a week or two.